Circuit/System Description
WARNING: This page is about a different car, the 2008 Cadillac XLR. However, it is still accessible from the selected car via links, so may be relevant.
The CD Changer supplies voltage to the steering wheel control momentary contact switches and monitors the return signal. Each switch state is associated with a set resistance value and when the switch is pressed, a specific voltage drops across the resistor unique to the switch. The CD Changer identifies the switch selected and activates the feature.