I presume it would be a similar setup to the electric windows. i.e. when the window or mirror is in a position where it can not move any further, the motor will draw more current.
A current sensing circuit will then sense this and shut down the motor. Thats how I would do it anyway!
If they used a timer, then if for some reason, maybe cold/ice, the window/mirror is slow at moving, then it would stop part way!
If the sense is the case, then may have to tap into wherever the signal runs from, could be the switch on the door - in which case all you have to do when you get an arm signal is to mimick this at the switch! (he says)
I could be wrong however - it has been known on more than one occasion!!