*Every* time this has happened to me pulling the bottom cover off the steering column and spraying DeOxit into the switch contacts has resolved the issue.
In my experience the switch contacts that carry good-sized loads all by themselves (wipers, turn signals, trunk latch, fuel door) oxidize enough over time that eventually they can't carry the full current load to the downstream motor or solenoid. This shows up as an intermittent fault, depending on what pieces of the two contacts are rubbing together that particular day... and sometimes if you fiddle with the switch it will suddenly work.
Might be worth a try before tearing apart the motor end.
Or, could just be a bad motor.