If I understand properly how the mirror works, make sure you have nothing blocking the sensor, which is located near center, on the other side. Also the mirror needs voltage to dim, it seems to me like intermittent electrical connection, could be wire itself, do the compass still show direction when mirror doesn't work properly? if the power supply doesn't work properly, exchanging mirror won't help. Also, if mirror stops working, is tapping on it any help? It would be easier for dealer to fix it if you could demonstrate that it is faulty and knowing how to make it misbehave at will. Any intermittent problem will disappear when the car is at the dealer, that's part of Murphy's law and I experienced it myself on more than one occasion.