WARNING: This page is about a different car, the 2007 Buick LaCrosse and 2007 Buick Allure. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: VCIM to Inside Rearview Mirror Schematic
Courtesy of GENERAL MOTORS CORP.
Fig 2: Power, Ground, Serial Data, Antennas, and Voice Recorder Schematic