How do VW wipers "know" how much rain is falling?

This is something that has amazed and intrigued me ever since we took possession of our VW Passat wagon: when I have the winshield wipers on “intermittent”, the wiper speed adjusts automatically to the amount of rain on the windsheild. More rain, faster wiper actuation. Less rain, wipers slow down.

How the dickens does the car sense the amount of water on the windshield? I can’t see any sensors or electrical contacts on the glass, but that doesn’t mean they aren’t there… Is it an optical sensor looking at light through the glass? some type of electrical conductivity measurement?

How’d they do that?

From here:

Wow, that was quick. Thanks Scruloose.

Trupa, off to look behind the rear view mirror.