OnStar® Microphone
WARNING: This page does not describe the selected car, but rather 6 other vehicles, including the 2002 GMC Yukon XL, 2002 GMC Yukon, 2002 Chevrolet Tahoe, 2002 Chevrolet Suburban, and 2002 Cadillac Escalade EXT. However, it is still accessible from the selected car via links, so may be relevant.
The OnStar®, or cellular, microphone can be part of the rear view mirror assembly or, on some vehicle lines, can be a separate, standalone unit. In either case, the microphone is supplied voltage on the cellular microphone signal circuit, while voice data from the user is sent back to the VIU or VCIM by means of either a cellular microphone low reference circuit or a drain wire.