Gm decided to stop allowing us to buy radio service manuals after the '94 model year because they wanted to lock up all the repair business for themselves. This is a common problem with changer mechanisms. Unfortunately you are tied to the dealer and their two grossly-overpriced repair centers.

In an attempt to prevent owners from buying high-quality aftermarket replacement radios, they started building the Body Computers into their radios so you had to keep them in the car. They began doing that nonsense on some 2002 or 2003 models. I don't know if that applies to your model.