I had this problem on my van (isothermic screen), I had to buy an external attenna, but only certain sat navs can accept these. The heated screen shouldn;t make a big difference. As mentioned by other posters, get the Quick Fix GPS thingy. It recalls your position and makes it pick up satelites quicker. The other thing to try is put it up by the rear view mirror, there is a 'clear' area.
04 plate Ink Blue 1.8 Duratec Ghia. Mods so far, club sticker on rear window and stereo upgrade, 4Q sticker on bootlid and Smurf hanging from rear bumper.
More mods to come as i\'m allowed to.