On my 82 300SD I have had several of the motors stop working. The problem I have had is that the bushing at the end of the motor shaft siezes with the motor shaft.
I have repaired them by disassembling the motors, which does do some damage to the motor, free the bushing from the motor shaft and work on it til it runs freely again. Basically cleaning, opening it up a few thousands of an inch and lubricating.
The motor is not made to be repaired. I have to force the bushing out and am only partly able to reseat it with a retaining clip. I get them to work, but use them as little as possible.
The only way to properly repair them is to replace the motor assembly. The assembly consists of the motor and gearbox. The cost is $100. +.
82 300 SD
98 ML 320