Circuit/System Description
WARNING: This page is about a different car, the 2008 Chevrolet HHR. 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 that switch. The radio identifies the switch selection and activates the feature. Otherwise the radio sends a serial data message to the component controlled by the switch and activates the feature.