Hi there. Definitely check the switch. I wasted a whole load of time checking all the wiring, the resistor pack, took the motor out and bench tested it. Nightmare getting the fan out unless you have child's hands! All OK. Took the switch out and tested with multi-meter getting no response at all and noticed a tiny bit of burning on the plastic next to one of the contacts. Went to the Ford dealer. Not one in stock anywhere in Leeds (proving how often they fail) so had to order it. Put it in this evening (15 minute job) and it sparked into life straight away. Cost the best part of £23.00 which is outrageous, but at least the missus is talking to me again now the heater's working (her car). Your symptoms sound exactly like mine so I bet its the switch. If you have a multi-meter, you can also easily take the resistor pack out and test it, but if the resistor had gone, and the switch was OK, it would still run on 4. My money's on that dodgy switch. Hope this helps. Chris.