There are limit switches inside the motor mechanism to cut the motor off when the worm gear reaches the end of the stroke. A previous owner might have disconnected it for that reason. If it's still functional you'll want to check those to ensure it doesn't burn out the motor before you try it with the normal wiring.
What I did was hotwire the system with a lawnmower battery and a couple wires. Pull the cover plate off the shifter mechanism (good idea anyway, no telling how long it's been since it was cleaned and what's growing in there, mine was gummed up pretty bad), then apply 12V to the wires in turn, the motor should run to the end of the shifter stroke, then check that the power cuts out. Replace the cover, add light oil to the fill plug hole, and it should be good to go. Fix the wires, go out and have some fun.
