No experience with the Mondeo but I have repaired many an electric heated seat in Volvo's, Merc's and Landrover and it's usually down to either a faulty relay (not common but still check) or broken wiring in the area where you 'slide' into the seat.

You can ussually check the backrest and seat seperatly for resistance (unlimited resistance being broken wiring).
Getting the leather of, soldering the broken wire with a thicker one and refitting the leather can easily be done in around one hour time.

I hate heated seats but I hate having them whilst not working even more, hence me fixing them lol