Normal Operation
WARNING: This page is about a different car, the 2005 Ford Pickup. However, it is still accessible from the selected car via links, so may be relevant.
The steering wheel audio control switches consist of a specific resistor value for each button function creating a voltage drop. When a button is pressed, a specific voltage is sent to the audio unit. The audio unit then uses this voltage to determine which control input function has been selected. Circuit 595 (LB/RD) is the reference voltage circuit between the steering wheel audio controls and the audio unit. The ground circuit for the steering wheel audio controls is through the audio unit, circuit 848