In Reply to: Stupid question posted by Jim Higgins on January 05, 2001 at 08:29:29:
It is a photo electric sensor to automatically adjust the brilliance of your check control and odometer readouts according to ambient light. You will also find that on the OBC (mkIV) there is one just at the top right hand side (next to the display). Yours may or may not be the mk IV.
To test whether these sensors are functioning as they should, shine a torch onto the sensor or put your hand or something over it. The adjustments to the display should be easily noticeable.