I've got a Blazer with rear defrost and rear wiper, with a big chevy bowtie on the outside of the window. It's just fine. Some of my friends thought it wouldn't work..... it does, and I still use the defrost and wiper.
Rear defroster is on the inside of the rear window and there will be no problem except the cold/heat will make the sticker material brittle and it may crack off after a while. Also if there is snow/ice in your area, scrape windows with caution!!!!! Also don't stick to the lines on the inside of the window because when removed, it will peel away the lines but the outside of the window is wide open.... JMO