Rear View Mirror Has 2 Sensors?

[winemart2]

So i bought a bracket -
http://www.ebay.co.uk/itm/191176740430

to attach the phone (satnav) to the rear view mirror.
Drove off in sunlight and the mirror went dark. I didn't know there was a sensor on the screen side of the mirror and the bracket covered it. So why would I want a dark mirror at night except when some *** blinds me from behind?

[alexp999]

It is a light sensor.

So the car can measure the brightness difference between the front and back.

If the car sees dark at the front, and bright at the back it will dim

With only a single light sensor facing backwards, it wouldn't know the difference between daylight and headlights shining through.

[winemart2]

DUH, why didn't I think of that :)