Dimmer Switch
WARNING: This page is about a different car, the 2016 Ford Fusion. However, it is still accessible from the selected car via links, so may be relevant.
The dimmer switch is a momentary contact switch that is integral to the headlamp switch. The headlamp switch is hardwired to the BCM. When the dimmer switch is pressed up or down, the switch completes a ground circuit to the BCM corresponding to the desired action (increase or decrease illumination brightness).