I can only speak for Cosworths as they have rad switches but on MK2 fiesta know they have stat switches as well as Sierras, would imagine FRST would be similar. If it's anything like the Cosworth the loom is useless and relays are known to melting amongst other problems as has been mentioned.

Have you got the car hot so the fans to the point where the switch closes, then tried performing a continutity test on the switch? Also at this temp you should get 12V out of the relay side and earth?

Would imagine will be switch or relay.

Kind regards,

Martin.