Circuit/System Description
WARNING: This page is about a different car, the 2008 Cadillac CTS. However, it is still accessible from the selected car via links, so may be relevant.
When a OnStar® key press is made, a serial data message is sent to the audio system to mute all radio functions and transmit OnStar® originated audio. After the audio is muted, the OnStar® signals are transmitted to the audio system on the mono audio signal out circuit, and returned to the module on the mono audio signal in circuit.