These motors fail just as often as the regulator but in my expierence they just stop working one day. Moisture and rust is always founds inside the motor if the gasket has gone bad (so if you do replace the motorput some silicone in it first). If your windows slow regrease the tracks with soap grease and play with some of the adjustments on the door (sliding bolts). If these dont help power the motor directly to rule out a high resistence connection. These cars are getting to the point where the wires in the doors are actually cracking.

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1981 300SD 512k OM603