It's very likely that it's the heater resistor, rather than the switch itself. This is located on the pasenger side under the dashboard. (You can get at it from the footwell without having to remove anything). Take a look on Ebay for heater resistor. It doesn't cost much. (I got mine from E-Ford they give you instructions). 10 minute job, you need a torx screwdriver.
Aled Williams, March 2006