Rus mentioned most of them fail due to a bad seal between the glass itself and the reflective coating. Water seeps in and reacts with the blue tint, causing it to rust. In removing my old mirrors, one piece of glass came completely detached from its backing, and the other was starting to come up. He mentioned that heating had nothing to do with it - glass without the blue tint does not rust. I figure if this pair from the new batch goes bad, I can have them replace again, and this time they will not be heated!