OnStar® Microphone
WARNING: This page is about a different car, the 2010 Pontiac G5 and 2010 Chevrolet Cobalt. However, it is still accessible from the selected car via links, so may be relevant.
The OnStar®, or cellular phone microphone, can be a part of the rearview mirror assembly, or on some vehicle lines, a separate, stand alone unit. In either case, the telematics communication interface module supplies approximately 10 V to the microphone on the cellular phone microphone signal circuit, and voice data from the user is sent back to the telematics communication interface module over the same circuit. A cellular phone microphone low reference circuit or a drain wire provides a ground for the microphone.