Start by checking the fuse for a remote amplifier. If one is used it may not be turning on or it could be not getting the turn-on signal from the radio. Sorry to say that GM has refused to sell us radio service manuals since the '94 model year. They want all that excessive repair business for their two grossly over-priced repair centers. For '99 models you can still try salvage yard parts to see if the radio or the amp is the cause of the problem. Starting on some 2002 models they figured out ways to prevent you from being able to switch those parts from other vehicles. Just one more way they've found to bleed money from unsuspecting owners after the sale.