My 1987 300 TD has a new problem. The fan for the climate control system works just some of the time. Recently I have tried to use the AC system, and the fan will not work in either low, auto or high-speed mode. Then when I go to park the car, the fan mysteriously begins to operate. At other times, the fan works fine.

Because, back in the winter, I got some funky performance from the defrost setting (fan would not come on but if I would jiggle the button for the defrost, it would) I am suspicious of the pushbutton control unit. Ideas about what else could be causing these problems?