Dimmer Switch
WARNING: This page is about a different car, the 2020 Ford F-150. However, it is still accessible from the selected car via links, so may be relevant.
The illumination dimmer switch is a momentary contact switch, integral to the headlamp switch, providing a non-networked input to the BCM. When the dimmer switch is pressed up for increased illumination intensity, and down for decreased illumination intensity, a ground signal received by the BCM on the corresponding terminal is interpreted as a request to increase or decrease the illumination intensity.