both side mirrors dim based on the same sensor on the rear view mirror. i found out the hard way by getting the side mirrors smashed by careless speedy biker. need to get mirror replacement is not fun especially BMW sells the entire mirror assembly and not just the mirror with the heating+dimming element. It costs over 2k for one side. Found mirror replacement on ebay for only a fraction of the price.
level of dimming can be affected with the kind of tint used on the rear windscreen and when you raise the rear sunshade, the mirror practically doesn't dim much unless there's a stadium of floodlights shining into your car.