Just had my rear washer go nuts too - Read through the thread - grabbed some tools and went to work.
Had a bit of moisture build up on the pcb cleaned it up but still had the same problem with the rear washer pump running continiously. New PCB time....
However being totally bloodyminded I took the board out of the cover, cleaned it again and dug a multimeter out..........
And the findings are.......
One resistor at the bottom of the board had water damage and had dropped from about 380 Ohms to 11 Ohms which instantly engaged the wash relay on the underside of the board.
Stole a pic and marked the resistor.
Removed resistor and all now works perfectly though one day I suppose I ought to get round to putting in a replacement.
Due to the location of the resistor I imagine this is the main cause of the washer going nuts so I hope it helps some people