if you have the connector for the screen aty the bottom, it may just be a case of getting a switch and finding the unused plug behind the dash as it should be there, probably need a relay in the fuse box aswell as yours probably isnt fitted with one,best bet is to get a focus with a heated screen next to yours and compare, you should find it easier to get the correct plug and work out what relay is different in the fuse box. HTH