WARNING: This page is about a different car, the 2007 Chevrolet Corvette. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Power, Ground, DLC, Audio Amplifier, Radio and Audio Signals (US8&U66) Schematic
Courtesy of GENERAL MOTORS CORP.
Fig 2: Speakers (U66) Schematic
Courtesy of GENERAL MOTORS CORP.
Fig 3: Power, Ground, DLC, Audio Amplifier, Radio, Audio Signals and Front Speakers (U65) Schematic
Courtesy of GENERAL MOTORS CORP.
Fig 4: Speakers and Noise Compensation Microphone (U65) Schematic
Courtesy of GENERAL MOTORS CORP.
Fig 5: Antennas except U2K, References to Navigation and Onstar Schematic