Circuit/System Description
WARNING: This page is about a different car, the 2008 Saturn Sky and 2008 Pontiac Solstice. However, it is still accessible from the selected car via links, so may be relevant.
The radio 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 the resistor unique to the switch. The radio identifies the selection and activates the feature. Otherwise the radio sends a serial data message to the component controlled by the switch activating the feature.