I've just scrapped off the overspray with a razor blade and some water/windex, it works really well and won't hurt the defrost lines like a solvent might. It didn't scratch the glass either, although you might test a small area to be sure. Of course you'll have to avoid scraping off the defrost lines themselves if the ospray is on the inside. If you wan't to remove it from the actual defrost lines, then you are a complete perfectionist that is in for a much larger ordeal.