Steering Wheel Controls: Function
WARNING: This page is about a different car, the 2007 Saab 9-5. However, it is still accessible from the selected car via links, so may be relevant.
The SID control module supplies +5V to the steering wheel controls via pin 25 and grounds them via pin 26.
The steering wheel control switches incorporate resistors connected in series. When a button is pressed, the activated switch is grounded, see the table.
The value of the total resistance in the circuit changes according to which switch is depressed and there is a corresponding change in the voltage which is sensed by the SID control module.