What's this sensor? (pic)
 

It's in the center of the bottom part of the nav screen.


http://img262.imageshack.us/img262/5...iche004zl2.jpg

I believe what you are referring to is a light sensor.

Yes it looks like a light sensor, but how does it work? I thought nav brightness is set manually under the SET menu.

Cabin dark...puts screen into night mode?

Relative bightness is set there, but during the day when it gets really bright out the screen can look washed out, so the intensity is increased/decresed automatically. Also as it gets dark out, you dont want the screen to be to bright, so it decreases the intensity.