1982 300SD wipers problem-

I have a 1982 300Sd that I am repairing for my kids to drive. I can hear the wiper motor running but the arms do not move? I know I have read somewhere about a Bakelite piece or something that wears out and it controls the arm movement. IS there a simple modification repair that can be done...AND how do I get to it, and how do I make the repairs.

thnaks

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