1983 Mercedes Benz 300sd right front seat

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This is more than likely a broken seat motor armature/cable. If you look carefully under the seat and follow the motor you should see an armature that leads off of it. Often times this armature breaks and often it is even visable as a broken cable under the seat. However sometimes the break is not as obvious. You described the motor as still running so the motor still works, only thing between the motor and the seat is the armature/cable that transfers the the power from the motor to the seat.