deville:
Iti is my understanding that the sensor senses cars behind you and auto dims the rearview mirror to cut the glare. somehow...

 

ok, i don't understand how, because the sensor is on the front, but ok... lol

 

the sensor on the backside of the rearview mirror detects ambient light levels, so that the mirror doesn't darken in the middle of the day when it's not needed. mine is really annoying, though, as it seems that streetlights and oncoming traffic's headlights are enough "ambient light" to keep my mirror from darkening, allowing glare from the vehicles behind the truck to blind me. had the same problem on my last dodge truck with this feature.

 

OK I didn't know about the one on the fornt bumper side of the mirror...
Maybe that is the sensor for the auto-headlights to know when it is getting dark.?

 

the ambient light sensor on the front side of the rearview mirror is a square hole, you can see it from outside the truck or feel it with your hand, and the headlight glare sensor is a dark spot on the reflective side of teh rearview mirror. the auto-headlight sensor is the small bump on the dash right up next to the windshield towards the center of the dash.