I have found that most times that I have had this problem it is a broken wire at the hinge of the door. The wires can only bend back and forth so many times before they break. I did have one window motor that had a frozen bearing and I took it apart and re lubed and it works fine. If you look at posts in my name there are pictures of the motor all taken apart. When you take the inner door panel off there is a terminal block where the wires from the switch connect to the wires to the motor and this is the best place to test the motor.

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1983 300SD
200000miles