The mirror stop position, should be controlled by some sort of limit switch.
so, id say you need to strip the mirror and find out if there is a micro limit switch for open and closed position. Closed position is obviously working so you need to either replace the bad switch or repair it to act the same as the working switch.
The should be some sort of ball and detent that it clicks into when it hits its position so look at this also when you open the mirror. It might not be putting enough resistance on the mirror to tell the motor to stop.
Hope these ideas
help let us know how you go....??