My Mondeo is a pain, when I really need the demister and switch it on to 4 the fan starts up with great gusto and the vents promptly close up so there is no output. As soon as you reset it by going to the off position and then back to demist you get a quick blast and then nothing (fan still belting away) any ideas would be appreciated. By the way it happens on all speed settings and in every distribution mode - I think a spring has gone or a damper falls out of place.
Its the pollen filter I think. I had exactly the same problem some time ago on my old Mondeo. Took the pollen filter out (under the cover beneath wiper arms) and it was clogged. It actually ran perfectly without any filter in at all but put a new one in and all OK. Hope this helps - even if it is a little late