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I haven't cycled my headlights for a few years and one opened halfway and then was "loose" and flopping around. I noticed that both motors keep running a while after they are closed or opened. Does this mean the Derlin gear is stripped?
Most likely you just need to replace the factory "goo" that dried up and failed. Repair can be as simple are installed a set of bushings to replace the goo.
Make your own or order these (3 bushings per headlamp motor):
My driver side would open up but it kept running for a while till it stopped itself. Passenger side was fine. I decided not to go with the bushing fix and go with the replacement of the gear and bushing. I also replaced the other side. The driver side was dust and the passenger side was ok but the stuff they put there had already elongated in shape.
The bushings will require some filing down to get it right. DON'T be tempted to file a huge bunch off. File a little off, insert it into the bronze gear and put the top piece on and make sure it fits. 2nd and 3rd one, do the same.