I had this problem and the coil pack fixed it. The problem however came back with a vengence the following year, again coil pack solved it. Apparently water leaks through the washer fittings and drips onto the pack causing the problem. I put sealent on the underside of the washers to see if that worked, but sold the car soon after so I can't give you a follow up! If you look into it, ford added sealing around the washers for the facelift version. The problem obviously didn't go un-noticed!
SO my advice, change the coil pack and talk to your dealer about fittings for the windscreen washers!