FixVault

OnStar Schematics

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: Antenna, DLC, Ground, Power and User Inputs Schematic
GM1410255Courtesy of GENERAL MOTORS CORP.
Fig 2: Audio Output Schematic
GM1410258Courtesy of GENERAL MOTORS CORP.