The dimming rear-view mirror has two light sensors. There's one on the back of the mirror housing, that looks out through the windshield to sense the ambient light level. The second sensor is on the side of the mirror that you can see, near the dim green LED. It senses the headlights of cars behind you. If there are bright lights that shine on the front sensor, they confuse it to think that it's daytime, and so the dimming action won't occur when headlights approach from behind. I've found that streetlamps (amongst other things) can confuse the front sensor.

Stan