Waggin

On mine, it has to be pitch black outside for it to start dimming. On rainy evenings, even a dark gray sky is light enough to prevent it from dimming, even when high truck lights behind me are shining directly on the mirror and into my eyes.
Unfortunately, there's no way to manually dim the thing.

Can I adjust the ambient light threshold where auto-dimming begins?

Theresias

As far as I emember, the dimming is beeing done through the sensor of the interior mirror as master.