There is a modification that was done to later models and can be done to all, The pollen filter housing is the most likely cause by all the trouble shooting you have done so far. If you have had a pollen filter change at anytime and bought an none ford part then the likelyhood of this being any good is minimal. The only way of getting the right filter that I am aware of is through Ford unfortunately. The modification to the housing is a frame glued in place and a foam filter added. There is also a check of the air valve and a different carbon filter and resistor pack fitted to the heating fan under the same mod. However it is always worth checking that the filter that you have got fitted is correctly seated first. I had exactly the same problem when I changed my filter for a none Ford part. It was good up until then and has been fine since an original Ford part fitted. Even though I don't like buying direct from Ford it would seem in this case it could work out cheaper than sorting out electrics and rust later on, that is besides the heavy mist on the inside of your screen in the mornings and even worse when it freezes