I would imagine if it's running off OE loom the fan switch went and then someone bodged it with a normal switch. Check the fan switch wiring, it's on the thermostat housing on CVH XR3's not sure about Zetec ones it may just be as I said and someone coulden't bother to sort the switch and just spliced into the wiring with a normal switch in which case all you need is a new switch but these don't usually give problems.

Check your battery to see if you have wires and a relay near it, this would indicate someone has attempted to rewire which isn't a bad thing to have done.

Martin.