fords are well known for switch failiure, pull out the heater switch keeping the wiring conected, with a test lamp find the feed to the switch, then with the switch turned on (ignition on) test all contacts to ensure power is coming out of the switch, i'd give good odds that the problem is within the switch. don't make the mistake of buying a scabby old switch from a breakers because the switches are rubbish to start with and it will fail soon after, however, if you find the switch works fine, try the earth at the heater box itself. one more thing, if you are having heater switch problems my guess is it will be you lighting next, your side and tail lights will vanish but headlights will work fine, likewise with the heater change the switch.
den harris
March 2006