WARNING: This page is about a different car, the 2003 Pontiac Sunfire and 2003 Chevrolet Cavalier. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Locating Cellular Communication Components (Inside Rearview Mirror W/UE1)
Courtesy of GENERAL MOTORS CORP.
Fig 2: Locating Cellular Communication Components (OnStar Cellular Antenna)
Courtesy of GENERAL MOTORS CORP.
Fig 3: Locating Cellular Communication Components (VCIM Assembly)