WARNING: This page is about a different car, the 2007 GMC Sierra and 2007 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Radio Power, Ground and Serial Data
Courtesy of GENERAL MOTORS CORP.
Fig 2: Amplifier (UQA) and Rear Seat Audio (UK6) Power, Ground and Serial Data