My 53 had the same thing. Tracked it down to moisture in the screen washer pump. You can test if this is the cause for you by squirting the rear windscreen for a few seconds. If it stops the wipers then this is where the problem is. Remove the pump which just pulls out of tank (you do have to remove the front right headlight first, nightmare) take the pump out of the plastic housing and let it dry on a radiator overnight. I sealed mine on re-assembly with insulating tape to prevent damp getting in again.
Twostage
March 2010
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