Just to add my 2 cents on the original question.
I can visibly see my side mirrors dimming at night when a car with bright lights is behind. I can detect an edging effect where the edge of the mirror stays as normal, but the main body of the mirror dims a little. Then you can see the difference in shading.
Maybe try this at night. My stupid idea would be to park up at night and shine a torch on the rear view mirror sensor to make it dim. Check the rear view dims, then check on the sides?