Alex-
You might check the aux. water pump located in the front right side (passenger side) near the head light. It is a little servo motor about 6 inches long, about as big as your fist. Its held in a rubber grommet. I replaced mine a week ago because it had frozen up and actually shorted out the push button unit. I had to replace the push button unit too.

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Ed
1985 300D w/ 196,000 miles