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 System Component View
Courtesy of GENERAL MOTORS CORP.
Fig 2: Vehicle Interface Unit (VIU) And Vehicle Communications Unit (VCU) Component View
Courtesy of GENERAL MOTORS CORP.
Fig 3: Left Side Of Rear Compartment Component View