WARNING: This page is about a different car, the 2003 Pontiac Aztek and 2003 Buick Rendezvous. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Rear Seat Radio Controls Inoperative Diagnosis (Steps 1-2)
Courtesy of GENERAL MOTORS CORP.
Fig 2: Rear Seat Radio Controls Inoperative Diagnosis (Steps 3-11)