I've never encountered any sort of electrical interference with a TomTom Satnav, whatever it's near, so I'd guess there is no problem - as long as the Satnav can "see" the statellites (sic) you'll be ok. It is a guess though
I know people had trouble with a reflective coating Renault were/are using but certainly heard nowt from Ford owners who have the heated screen and satnav. Hope there are no issues as I have ordered both
I have the heated screen and a Tom Tom Go 500, never had a problem with the Sat Nav connecting up - it is suckered on to the Window - but comes out as far as where the Port would have been if I ordered it ;-)