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I'm currently in the process of upgrading the speaker system in my 2003 Corvette. The plan involves installing a 3-way system with two 6.5-inch woofers and two 3.5-inch midrange drivers in the doors, surface mounting the tweeter on the interior A-Pillars, and replacing the rear speakers with two coaxial ones. The power for this setup will come from a 5-channel amp located in the back of the car.
To maintain a clean and organized wiring layout, my aim is to utilize the existing wires leading from each door speaker to the stereo's harness. As far as I understand, each door's woofer has its own individual amplifier, but there is only one set of signal wires that travels to the stereo's harness. What color are those wires, what guage are they and do they run directly to the stereo harness? I'm not concerned with the color wires associated with the Bose DSP, as I'm completely removing the Bose system. Once these wires reach the stereo's harness, I plan to extend them underneath the center transmission panel all the way to the amp in the back. The speaker signals will then be transmitted through RCA cables from the new head unit's rear preamp outputs to the amp in the back in the same manner.
In the "C5 General" forum you will find a sticky at the top of the page labeled "Service Manuals," or some such. Download the zip file and you will find that it also includes the wiring diagrams.
bit of advice -- I went through this, and the wiring to the doors is kinda crappy and really thing (12 gauge? 14 gauge? I forget, but it's not what you'd consider quality), and at some point goes through a DSP (I forget if the DSP is integrated into the ..ehm.. "amp" strapped to the back of the paper plates .. uh.. "subwooffers" in the door or elsewhere). I suggest just running brand new wiring, especially if you're going to be replacing everything... just no point to keeping what's there.
It's about as easy as can be, the grommet is right there. I've done a bunch of backyard installs on multiple cars, and surprisingly, the c5 is really easy to run new wires and make it invisible. your best bet is to go from the door speakers, out the grommet, under the door sills on each side into the back to your new amp, and just forget the gm wiring. Using the existing wiring, and splicing into it to "extend" it is kind of a hack job, especially given you're replacing everything.
*maybe* keep the wiring to the back speakers. Maybe (but the location for those speakers are so horrible they add nothing, I routinely fade mine out completly)
I just finished an install of all new speakers and and amp upgrades, I didn't reuse the factory speaker wires,but during my research I watched some youtube videos of someone that did that explains the speaker wire colors and good install overall. Here is a link to one that was good.