You can test the mirror to see if it is dimming correctly.
1. take a piece of electrical (black) tape or duct tape and tape the square sensor behind the mirror.
2. a shop light or something really bright and shine into the mirror from inside the car.
3. Wait a few seconds and the mirror should dim if it is working correctly. If not, then check the fuse and if all fail, bring it to the dealer check it out.