Circuit/System Description
WARNING: This page is about a different car, the 2008 Chevrolet Malibu Classic. However, it is still accessible from the selected car via links, so may be relevant.
The radio supplies 10 volts to the audio steering wheel control multiple momentary contact switches, and monitors the return signal. Each switch is associated to a set resistance value, when pressed a specific voltage drop occurs through a series of resistors unique to that switch. The radio will perform the desired function, or send a serial data message to another component.