One of the things I discovered early in the process of working with the C6 stereo systems is that there is line level signal available in the factory harness in ALL of the systems, Delco and Bose and including NAV. This is true of ALL of the model years of C6 through 08 and I expect that it may continue throughout the generation, though that is speculation on my part.
This harness you create will become a "Y" adapter, with one side of the Y being the factory connector and the other being RCA outputs to hook to an aftermarket amplifier to drive a full system or even just a subwoofer. This is the BEST way, hands down to get this done. You do NOT need a Cleansweep and using the full PAC AOEM-GM24 adapter adds cost and worse, adds an ugly EQ curve that rolls off bass that will hurt the end result of your efforts.
So, enough pre-amble, here's now to make it...
Parts you need:
RCA Jack Red Plastic Part Number 090-266
RCA Jack Black Plastic Part Number 090-265
These are recommended vendors and parts, you can use any female solder-on RCA's you care to use, Radio Shack has them as well. The ADD-GM24 is the only part that is not optional, and though you may find it elsewhere the easiest place I have found to get it is Sonic Electronix.
You start by cutting off the connector that is intended to plug into the evil OEM-1 box that induces the aforementioned nasty EQ curve. These are the wires you will solder on to the female RCA's. Here's what it looks like:
Solder on the connectors using this table of wire colors:
Left Front + White
Left Front - White/Black
Left Rear + Green
Left Rear - Green/Black
Right Front + Gray
Right Front - Gray/Black
Right Rear + Purple
Right Rear - Purple/Black
The RCA connectors will have a small lead in the center of the plug which is the positive(+) lead. The longer connector is the negative/common(-) lead. Be patient here, and you may want to buy a few extra connectors cause the leads are close together and it's easy to get sloppy and have to start over. The key is to make sure that the leads & wires of each connector do not make contact with one another, if they do the connection will not work. Also, make sure you slide the screw-on cover over the wire BEFORE you solder on the connector!!!
When you're finished your harness should look something like this:
I hope this thread answers some questions and makes clear what you need to do in order to get the best possible output to feed an aftermarket amp while keeping the factory head.