If your amp has speaker level inputs, you can just tap into the rear speaker wires. Make sure the negative on the amp's inputs are not grounded though. I think most newer GM radios use a floating ground output scheme to drive the speakers. That means if you ground the negative coming out of the radio, it won't like it for long. Poof. You could also get one of these line level adapters. I used one of these in my Blazer for several years. That radio is probably about the same as yours. Just connect the inputs to the rear channels and it's RCA outputs to the amps RCA inputs. This one has level adjustments also. I got it at Crutchfield I think.
David
edit: Crutchfield has a couple listed, search for "level converter". A Best Buy or Circuit City type place near you might have some too.