For 1990-2009 cars

The audio steering wheel control switches duplicate the function of the primary controls of the associated component, through a network of multiple momentary contact switches. The radio supplies voltage to the 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 the switch. The radio identifies the switch selection and activates the feature. Otherwise, the radio sends a serial data message to the primary component controlled by the switch to activate the feature.