My mondeo mk2 was doing this easily solved.
remove passenger wiper arm. then remove 2 screws holding the black plastic trim between window screen and bonnet. remove trim.
now you should see the pollen filter housing.open it by the 2 clips one each end and take filter out.
now undo the 2 bolts to remove the back of the filter housing.
you will notice the seal is probaly gone.
clean the area around where the housing meets the car and clean the housing.
put a good layer of silicone sealant on both the car around the lip you will see and on the back housing unit. carefully place housing back in place and tighten both nuts up tight.put extra sealant on as reqiured and smooth out with finger to make a good seal.allow to seal and reassemble everything. good idea to replace pollen filter now.
Terry
March 2014