The wiring diagram says its a solid state controller, and the connections from the switches indicate that the switches apply a control signal to the box and the box then applies power and ground to the motor. The controller could probably be removed and disassembled, but it would be easier to check the connector to the motor first. Disassemble the connector and check the two large sockets. Make sure the wiring is solid. You might try squeezing the sockets down a bit to make a better connection with the pins on the controller.

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)