WARNING: This page is about a different car, the 2006 Pontiac Montana SV6, 2006 Chevrolet Uplander, and 2006 Buick Terraza. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Ground, Power, Serial Data & Front Audio Signals Schematic
Courtesy of GENERAL MOTORS CORP.
Fig 2: Rear Audio Output Schematic
Courtesy of GENERAL MOTORS CORP.
Fig 3: Auxiliary and Rear Audio & Video Signals Schematic