Any issues I should be aware of with placing a window decal on the back of a car with defrosters and a windshield wiper? Will the heat from the defroster cause the vinyl to loose adhesion? What about the windshield wiper going over the vinyl?
defroster wont be an issue with proper vinyl. wiper on the other hand is going to beat it up and tear it eventually no real way around it. try to use the wiper sparingly.
Just throwing my $0.03 in here. I can't speak about cold+wiper, but here in FL the rear wiper on my truck can be pretty busy when the weather decides to kick up. It doesn't have to be raining, either. Apparently the Explorer's body is adept at sucking spray off the rear wheels into the window. For the rest of the year the sun is particularly merciless.
While I didn't cut the last decal that was mounted there, I know it was done with Oracal 651 and I got a good 4yrs out of it before it really showed signs of wear. I'm sure freezing temps and road salt will be quite brutal (and I know from living up north that ice chippers are lethal to decals).
YMMV but I'd plan on using the best vinyl you can afford, sealing it if possible and generally trying to care for it as much as possible.
Back window lettering on a car???? 
