I just pulled out the rear wiper assembly on my 300DT 1983 wagon. Have not fully diagnosed the problem, but here is what I found so far:

• The rear wiper switch seems to be working. I measured 12V between pins 3 and ground at the rear wiper.
• Motor does not turn. Upon opening the gear box assembly thingy, I noticed the main plastic gear (white with lots of brown grease on it) has copper terminals with shapes cut in - I suppose this is to reverse the motor direction as the gear turns.
• Voltage to the gear is supplied by three terminals that are fed by wires on the outside of the assembly housing.

I wanted to know if anyone else has attempted with fix before and knows what to do here?