I recently did a 2002 tahoe with a similar problem. Is On-Star an issue? My customers truck had OnStar but he didn't have it activated, so I didn't pay 2 cents to it and replaced the controls with the lil pocket non-OnStar trucks came with.
hottwhyrd is right on the money, the amp is behind the glove box. You can throw in your four channel, or use a standard GWH-344 plug and patch the speaker wires with rca's. Now you can just hook up the rca's to your HU without messing with speaker wires.
Is this the way for you to go? I don't know, but its food for thought...Good Luck!