some heated front screen switch off after a while, im sure my wife escort does, cant remeber about my fiesta as i had to change the screen a while ago cos i cracked it and it was cheaper to get a standard one from scrappy.
find what relay you need and let me know as i have a few floting about in the shed, other than that get down the scrappy.
anyone know for sure what car does? i got over 100 haynes manuals mate, im sure i could find a relay for the job out of that lot, lol

as for the extra current draw, i can see it upping the revs ( may be wrong) but what about old cars with heated rear screen and no ECU, how does the revs pick up then!
i think someones pulling ya chain dude, if you were to switch every thing on you car still runs dont it, i wouldn't worry to much about the revs thing TBH