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Its a piece of string question, there could be a number of reasons.
check the switch first, then pop the mirror glass out and look at the wiring on the motor.
After that, its trace the cables in the door, through the door grommet and back to the switch.
If you want a stab in the dark........mirror motor corroded !
Mines tempermental too on one side.. and i assume mines a broken wire as i tried the switch and it made no difference.. you could get a multi-meter on it and see if its getting power to the switch.. or as been said pop the mirror out and see if its a wire there.