97 Mondeo; Intermittant heater fan fault (usually after it rains!)?

I have a 97 Mondeo that displays an intermittant fault with the heater blower fan.
It always works on position 4 but doesn't always work on 1,2 and 3.
I have already changed the resistor which temporarily solved the problem. However, whenever it rains heavily or I wash the car, it stops working on 1,2 and 3 again and takes a couple of weeks before it works OK again. Any ideas?