I have installed a Bose CD deck in my 1999 FRC. Originally the car had the base cassette unit and I wanted to update to a Delphi/Bose CD deck. I had wanted to use any old Delco or newer Delphi CD deck that would fit, but then certain radio functions controlled by the C5 BCM, like display brightness, SCV and retained power would not work. Older Delco decks without full BCM remote control will not work correctly. Delphi, GM, and everyone else, told me it was impossible to upgrade a FRC that came with the base cassette unit to the "Bose" CD deck, it is not. There are possibly other newer Delphi decks that will work, but it is easier and cheaper to get a C5 deck from the forum. Without Delphi giving up the true info on the C5 "Bose" deck, it is risky to buy another Delphi CD deck. Bose does not make the head unit.

Bose CD/Cassette Stereo System Info:

The C5 Bose stereo system is based on a modified Delphi head unit. The "Delpi/Bose" head unit, CD or casssette, has connectors called C1, C2, C3 and C4 on the rear panel, plus the antenna. C1/C2, which is a single siamese connector, is used for all Delphi head units, not just Bose. C1/C2 carry power, ground, PCM/BCM data and the front and rear speaker wires. Connectors C3 and C4, provide interfaces to the head unit front/rear pre-amp outputs and front/rear power amp inputs. The pre-amp's audio outputs on connector C4 go to the BOSE Signal Processor, BSP, mounted above the throttle. The BSP "conditions" the front/rear audio signals and then sends them right back to the head unit internal 4 channel power amp. The basic crossover frequency of the BSP is 1 kHz. The BSP also does ambience and spatial enhancement to the audio signals. The internal power amp is about 15-18 watt rms/ch. The head unit power amp drives the 4 front and rear Bose midrange speakers called Twiddlers. The rear Twiddlers are 5.25" and the fronts are 3.5". The Bose system has no true tweeters. In addition, the BCP extracts the low frequency information from all 4 pre-amp input signals and sends them to the two sub-woofer amplifiers in the doors. These sub amps feed two reverse cone 8" "sub-woofers" in the door. They are mounted next to the front 3.5" Twiddlers behind the door speaker grills.

All low level audio interfaces in the Bose system are psuedo-differential. These include pre-amp, power amp, subwoofer signals and CD changer. All high level speaker outputs, head unit and sub-amp, are fully differential. A psuedo-differential wiring scheme is one that shares one return signal wire for many signal source wires. A differential wiring scheme uses two wires per signal. Using psuedo-differential or true differential wiring for low level signals improves noise rejection. Pseudo-differential wiring costs less than differential wiring and does not reject noise as well. Using differential wiring for high level signals improves voltage swing by 2x and thus power level. 2x the voltage swing is 4x the power. A typical single ended power amp powered by +14 volts, will put out 4 watts. A typical differential or balanced power amp running off +14 volts, will put out 16 watts. NEVER ground any psuedo-differential or differential signals to chassis. You may damage the internal differential circuitry. There is a dedicated floating Common return signal for each low level interface. DO NOT use the Common signals to interface to any external equipment. The Common signals in the C5 audio system are non-standard and should only interface to Delphi or Bose equipment. The 4 Common signals on the head unit and BSP have a DC offset on them. If you ground any of them you may do damage to internal circuitry. Only use a high impedance interface to tap a pre-amp outputs to feed external amps. For a power amp input signal return, use the shield wires or chassis. Tapping a differential signal with a single wire is OK, but you will not get the same noise rejection on long cable runs.

FRC Base Cassette Stereo System Info:

The FRC base or non-Bose cassette head unit is similar to cassette units in other GM vehicles. It is VERY different inside compared to the Delphi/Bose cassette head unit. The FRC base cassette deck internal circuitry look like a much newer design. It shares very little circuitry with the Delpi/Bose unit. Even the tuner is a much newer design. I have not compared what tuner is superior. The FRC base deck utilises the same C1/C2 siamese connector that the upgrade Bose head unit does. All power, data and speaker connections are the same. In fact all C5's have the same wiring harness for all stereo options, you just use two less connectors, C3 and C4, for the FRC base cassette unit.
The FRC base system has crap GM 6.75" front speakers in a FRC only door cutout. The fronts are pathetic $2.00 GM POS speakers. It takes real ***** to put these cheap speakers in a Vette. I have seen better speakers in a $20 boombox. The rear speakers are the same Bose 5.25" Twiddlers in the Bose upgrade. The rears are pretty good quality, but have no bass or upper high frequency response. They are basically mid-range speakers. You could just add additional tweeters in parallel with the rear speakers but you will have to cut holes. While the rear speaker is a standard size, the fronts are not. The only speakers I found to fit the FRC door cutout exactly is the 3-way Pioneer TS-A1765. They are considered a "oversized GM 6.5 inch". To me they sound very good, with exceptional bass for such a small speaker. The door speaker enclosure is very well designed. I also put a 2-way Polk 5.25 EX502A in the rear. They sounded Ok, but are bass challenged and the tweeters don't point at the occupants. Should have bought another speaker. I wanted the 3-way Pioneer TS-A1365 for the rear, but did not want to order them. Always make a template of your old speaker before buying new ones.

FRC Upgrade instructions for the Delpi/Bose head unit:

The basic idea behind the Bose upgrade is to connect the head unit pre-amp outputs to the power amp inputs. I cut the two unused C3 and C4 connectors out of the harness with 3 inches of pigtail. The C3 and C4 connectors are unused, taped in a bundle behind the head unit. I used these pigtails to make a jumper cable. Below is the mod details.

The following are the Bose head unit 4 channel pre-amp outputs
Signal CD/Radio Color code
LF out C4(9) TAN/BLK
RF out C4(10) LT GRN/BLK
LR out C4(11) BRN
RR out C4(12) DK BLU
Common C4(13) LT BLU, DO NOT USE FOR EXTERNAL POWER AMP
Shield C4(14) Bare wire
Ground C4(15) BLK

The following are the Bose head unit 4 channel power amp inputs
Signal CD/Radio Color code
Common C4(7) YEL
Shield C4(8) Bare wire
LF in C3(1) DK GRN
RF in C3(2) LT GRN
LR in C3(3) BRN/WHT
RR in C3(4) BLK

Connect the following color wires together. Please check the wires per the info above.
There is two BLK wires, one is a signal wire and the other is connected to chassis ground, C3(4) is the one you want. Solder the connections and tape them up. Do not cut unused wires, you may want to add a power amp later and you will need these for signal returns.

C4(9) TAN/BLK connect to C3(1) DK GRN
C4(10) LT GRN/BLK connect to C3(2) LT GRN
C4(11) BRN connect to C3(3) BRN/WHT
C4(12) DK BLU connect to C3(4) BLK
C4(13) LT BLU connect to C4(7) YEL

Delphi/Bose head unit internal improvements:

These mods are not to be done by someone without soldering experience with surface mount components.

The Bose CD deck with the FRC bypass will not drive it's internal 4 ch power amp to clipping on some radio stations or selected CD's. It is not a real problem, but some old CD's were recorded at very low levels. These old CD's usually sound like crap because old tapes were used as the source material and there is a lot of hiss on them. If you like your music loud, then you will need to modify the head unit power amp for more gain. Most head units can overdrive their power amps to clipping for even low level inputs. I have a fix to increase the gain of the internal power amp. You need to replace four chip resistors. The mod requires changing out four 1206 size surface mount chip resistors. The head unit power amp has 4 op-amp based attenuators feeding it. Lower the attenuation and the power amp gain will go up.

The Bose head unit has a modified bass control to work with the subwoofer. When put in a system without a sub-woofer, the control does very little. A normal bass control has a turnover frequency of about 1khz. The Bose HU control is under 200 hz. The stock speakers in the FRC cannot reproduce the sub frequencies. I modified my bass control to work like standard Delco deck. The change requires changing out some surface resistors and capacitors. This mod improves the bass enormously.

I have also gone through the circuitry and improved the low and high frequency response of all the audio circuitry. These changes required increasing the coupling capacitor values by 2x to 10x and eliminating some shunt capacitors from the C3/C4 interface.

Head unit gain and bass control modifications:

The resistors and capacitors I changed are on both sides of the head unit motherboard. I will only describe the gain and bass control mods for now. The other mods I did are too much for most people to attempt. The other mods only enhance the bass and highs more. This is something you will probably not need to do.
DO NOT force anything. Be patient.

To get at one side of the motherboard, you need to remove the radio circuit card, this is easy. To get at the other side requires removing the CD assembly. It is fastened with four screws. This takes more time because you must remove the front panel to get two of the screws. Remove the big ***** only and the nut and washer on the volume control. Be very careful removing the front panel. It is plastic and is clipped onto the metal chassis in many places. Work each "clip" free individually. Be careful removing the CD assy. There is a long ribbon cable attached to it. You must release the tiny compression clamp on the ribbon connector on the CD assy motherboard, to pull out the ribbon cable free.

The four resistors to changed the power amp gain are near the two SO-14 and one SO-8 IC packages used for the pre-amp output and power amp input buffers. The two 14 pin IC's are quad op-amps and the 8 pin IC is a dual op-amp. The resistors are clustered around one of the 14 pin SO-14 package IC's next to the C2/C3 connectors, under one of the RTV blobs. You need to remove the RTV blob on top of the IC. Leave the rest alone. The resistors are labled 5491. This means 5.49K ohm 1%. You need to replace them with four 18K to 27K ohm, 1206 size chip resistors. The larger the resistance value, the higher the power amp gain.

To mod the bass control circuit, locate the small 32 pin PLCC packaged tone control IC near the large Microprocessor IC. The bass control's 2 resistors and 4 caps are on the other side of the motherboard under the tone control IC. Look for two 1206 chip resistors labled 302 that connect to ground. This means 3K ohm 5%. There are four caps connected to these two resistors. It is a "T" network. For the R+L channels, two caps attach to the IC and the resistor hangs between the two caps to ground. The two resistors need to be replaced with 4.3K to 5.1K 1206 size chip resistors and the caps replaced with .1uf 1206 size chip caps.

Be very careful removing the old parts. Heat up both end terminations of a the chip component at once with a soldering iron. Use some extra solder to help transfer heat and work the tip back and forth between the ends. Make a small solder ball if neccessary. Gently push the part off the pads with the soldering iron tip when both terminations are floating free. Clean up the board pads with solder wick. Clean the board with a Q-tip and some alchohol or acetone. Use tweezers to hold the new parts to solder them.

 

Lotta work for something that's still a Bose. i'd rather listen to a walkman than anything I've ever heard from bose home or car.
92TripleBlack

 

It is not a "Bose" system just by putting in a Delco/Bose head unit. Bose does not make ANY head units. The C5 Delco/Bose head unit is just a variation of Delco head units used in other GM vehicles. The audio quality at the pre-amp outputs of the Delco/Bose head unit is as good as any aftermarket head unit.

This is the cheapest and fastest way to add a CD head unit to a FRC C5 with the base cassette head unit. It is easy to buy a used head unit off the forum. Many people have done this mod to add CD to their C5.

In the C5, the BCM controls the stereo. Using the C5 Bose head unit, retains BCM controlled features like illumination, SCV, favorite channels and retained power. If you want to improve the sound further, then add new speakers. I went further and modded the head unit to make it function like my Chevy C2500 truck premium stereo system which I like. You could use a Delco truck head unit, but you would have to cut up the wiring harness and you would lose the BCM controlled functions.

I think the "auto tone" style Delco head units used in the C5, Camaro, trucks and many other GM vehicles has the best ergonomics of any head unit out there. I hate most of the after market head units. They are a total PITA to operate while driving. I HATE buttons. I LOVE *****. I can operated my stereo without ever looking at it using one finger. You cannot do this with any aftermarket head unit I have even seen. I constantly adjust the volume control and change FM stations in between shifts.