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.
The body control module (BCM) supplies voltage to the audio steering wheel control momentary contact switches and monitors the return signal. Each switch state is associated to a set resistance value and when pressed a specific voltage drops across a series of resistors unique to that switch. The BCM identifies the selection, then sends a serial data message to the component controlled by the switch and activating the feature.