Volkswagen: overhead flipdown panel, rear view mirror, lighting panel

Question
I have a 99 VW Golf.  In front of the overhead lighting panel there is a small fold down panel, about 1 inch by 6 inches.  It looks like a sensor is located in the center of it.  Do you have any idea what it is or used for?  It is just above the inside rear view mirror.

Answer
i think it is either a rain sensor on certain models, or a light sensor (if fitted with an auto dipping mirror) the panel is a joining piece for the sun visors. ie put both sun visors down and the flap down and it virtually blocks light coming between the visors above the mirror. also on some models the alarm ultra-sonic sensor is located in the light housing. sorry i can't be more specific.