Wodeshed

Hi, everyone. I've got a problem I haven't seen described elsewhere. My w209 CLK rearview/driver's side mirrors seem to think it's night even in broad daylight. I don't have the gold tint/leakage problem others have described; mine seems to be the opposite. I can usually get the rearview to go back to daylight mode by squeezing HARD and releasing very slowly, but the mirror always dims again soon thereafter.

Does anyone know if there's some sort of light sensor that has malfunctioned, or have other suggestions for repair? The car is awesome otherwise, and looks new despite its 135k miles.