The problem is most likely to be the heater control valve has failed, easy to replace, it's at the back of the engine bay, follow the two smallish (1" 25mm) water pipes from the engine, there you will find the heater control valve, it has the two pipes you followed plus two more that go to the heater matrix, disconnect the pipes and the connector then it will un-bolt, refitting is the reverse. You will then need to top up the water for any that was lost.
If you would like the Ford fault finding guide for the heater system contact me on nige110162(at symbol)hotmail.co.uk
nigel, May 2010
