the side mirror dimming relies on the interior mirror dimming, it uses the sensor from the inside mirror. if that mirror isn't dimming, the sides will not.
if your rear window is tinted you will have issues. I have 5% on my rear window in my G80 and the mirrors will not tint, I had to invent an over ride to get the side mirrors to tint, the interior mirror isn't an issues for me as the rear tint blocks the blinding light, but the side mirrors/windows don't.
I purchased a laptop camera shield from amazon and stuck it over the sensor at the rear of the inside mirror (the white sensor that faces the windshield) when I want the mirrors to tint I reach up and close the shield. Works like a charm.
My mirror tint worked fine before I tinted the rear window and having experienced the same issue on my last dozen cars, I know it is the tint in my case.
here is my work around:
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