Circuit/System Description
WARNING: This page is about a different car, the 2008 Cadillac STS. However, it is still accessible from the selected car via links, so may be relevant.
The vehicle communication interface module (VCIM) provides the cellular microphone, located on the inside rearview mirror with a supplied voltage on the cellular microphone signal circuit. When the microphone is in use, voice data from the user is sent back to the VCIM by means of the drain wire.