The armature in the motors move fwd/aft as they turn, and hit the "limit". At that point, it is supposed to open a set of contacts, and kill the power to the motor in that direction.
Yours is either too loose inside(too much movement), or the contact switch is no being opened. Either way, it's not an external fix....the motor will have to be opened and repaired, or replaced.
btdt...several times...
Also...what has most likely happened is the gear(s) stripped...but that only happened because the motor is not shutting off when it should.
If the motor you put on is new, or one you bought from a company, then it needs to go back to them, and have them replace it. If it is a used one from another car, you can rebuild it yourself, if you have the patience. I've done mine about 3 times now, but one of the problems with mine is the magnets for the motors are not epoxied to the outer case like they should be. I've tried JB Weld, but, apparently, it don't work very well! So...after the headlights go up/down a few times, the magnets spin, and the motor quits.
Adams' Apple2009-03-20 19:52:50______________
Joel Adams
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"