OnStar® Microphone
WARNING: This page is about a different car, the 2008 GMC Sierra, 2008 GMC Cab & Chassis Sierra, 2008 Chevrolet Silverado, and 2008 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
The OnStar®, or cellular microphone can be a part of the rearview mirror assembly, or on some vehicle lines, can be a separate, stand alone unit. In either case, the vehicle communication interface module (VCIM) supplies voltage to the microphone on the cellular microphone signal circuit, while voice data from the user is sent back to the VCIM by means of either a cellular microphone low reference circuit or a drain wire.