Remote Radio Audio Signal Inputs
WARNING: This page is about a different car, the 2008 Cadillac DTS. However, it is still accessible from the selected car via links, so may be relevant.
Audio output from the OnStar® communications module connects to the remote audio signal inputs of the radio. When the cellular telephone mute signal goes to 0 volts, the radio over-rides any other audio signal and uses these inputs as the source for the output to the speakers.