Auto rear view mirror

I know this has been posted before, but not with the answer to the question I am asking,



How can you test if the mirror is dimming? I notice that there is a light sensor in the top edge of the mirror glass and also on the back of the mirror (facing forward) I tried shining a light into the sensors and nothing seems to happen, there does not seem to be any obvious dimming of the mirror, any ideas?

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