WARNING: This page is about a different car, the 2003 Pontiac Grand Prix. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: OnStar Schematics (Power And Ground)
Courtesy of GENERAL MOTORS CORP.
Fig 2: OnStar Schematics (Vehicle Interface Unit (VIU) To OnStar Keypad And Radio)
Courtesy of GENERAL MOTORS CORP.
Fig 3: OnStar Schematics (Vehicle Interface Unit (VIU) To Vehicle Communications Unit (VCU))