I had a sticker on the back of my last car for 3 years. And it got faded from the sun but I never had any troubles with the defroster either on the sticker or the rate of defrosting the window.
No. The only thing that will affect the decals is extreme heat (hot summer days) and extreme cold (winter). My Canadian flag decal has shrunk and bulged out a bit because of the temperature shifts here in Michigan.
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