It is a light sensor for your headlights
If you set the Switch to AUTO, the lights will stay off until you reach a shaded area, like bridges, tunnels, or even driving into garage, then the light will automatically turn on, until more light strikes that sensor.
In my business, i am required to have my Headlights on while i am on a driving job.
I have my headlight switch set to Auto & a bottle cap covering that sensor.
My lights remain on, unless the engine is shut off
Low Tech method of having running lights
In California & some other states, you will see these signs all over on sections of the roads
Those areas usually have signs reading Safety Corridor
