Seems like quite a common problem.

I had the same problem and have just changed the resistor as suggested and it now works.
Very easy, 10 mins and no special tools.

Heres how.

Tools.. screwdriver and torx driver


remove lower plastic below steering wheel, 5 screws..


Crawl into the footwell and look up to the middle of the car, clutch pedal is a little in the way..




Look for the resistor (It's the hole as I had already taken it out before taking a photo!)
and undo the 2 torx screws(careful not to lose them).


pull the resistor down..


and change it over.

I got mine off eBay for cheaper than a second hand part !!
http://www.ebay.co.uk/itm/HEATER-BLO...ht_1644wt_1195
HEATER BLOWER FAN MOTOR RESISTOR for Ford
GSF Carparts





hope that helps.