the front sensor allows the mirror to know its dark out. the problem is when you get too much ambient light in the front like i said. it still dims but not as much as im used to. manual mirrors are very dark all the time. the front sensor is the base light the mirror uses to decide to dim or not and how much to dim. the rear takes in headlights. a comparison is made and dimmed accordingly. if you put tape on it, then it would dim in daylight and your direction/temp display would dim also.