WARNING: This page is about a different car, the 2002 Pontiac Grand Prix. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Radio/Audio System Schematics (Power & Ground)
Courtesy of GENERAL MOTORS CORP.
Fig 2: Radio/Audio System Schematics (Radio to Speakers (w/o U85))
Courtesy of GENERAL MOTORS CORP.
Fig 3: Radio/Audio System Schematics (Radio to Amplifier (w/U85))
Courtesy of GENERAL MOTORS CORP.
Fig 4: Radio/Audio System Schematics (Amplifier to Speakers (w/U85))