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 radio is supplied with OnStar® a mono audio from vehicle communication interface module (VCIM). When the radio receives an audio signal from the VCIM the radio mutes the current audio signal and delivers an audio signal from the OnStar® system.