Rain Sensor
it's a very rough description...i'm sure others will give a better one..
Anyway here goes:Firstly, read the manual; all you need to know is in there - though it clearly was not written by Shakespeare it is somewhat informative

The sensor only operates in the "intermittent" wiper switch setting. Basically, it replaces a typical manually-variable intermittent rheostat. It DOES NOT operate the wipers when the switch is off.
The sensor itself is below the rear view mirror on the windscreen surface - it looks like a little fisheye lens.



