If it's a Delco system (not a Bose or not a Monsoon) the amp is on the underside of the glovebox and has a single 24 pin connector on it. How many speakers do you have in the rear, two or four?
Edit: Check your build label in the trunk for the radio code. Look for 'UX7' or 'UQ3'. That will tell us if you have an amplifier.