The valve is the fault. Accessed from under the bonnet at the centre of the bulkhead. Remove rubber seal strip. Pull away all wiring and pipes which run along the bulkhead after removing all plastic trim and wipers. Its low down on the metal landing of the bulkhead and most of the plastic clips will break as cars are made of the cheapest materials. use new jubilee clips and Blue Hylomar sealant when fitting the new part which even Ford were disgusted that they made such a poor quality part when I bought the last one! Dont bother buying new clips and getting it all back "perfectly" as Ford recon it could fail again in appx 15 months! Best of luck, it will take you about 45 mins if you are good with cars.