OnStar® Microphone
WARNING: This page is about a different car, the 2009 GMC Sierra, 2009 GMC Cab & Chassis Sierra, 2009 Chevrolet Silverado, and 2009 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, a separate, stand alone unit. In either case, the CIM supplies approximately 10 volts to the microphone on the cellular microphone signal circuit, and voice data from the user is sent back to the CIM over the same circuit. A cellular microphone low reference circuit or a drain wire provides a ground for the microphone.