I have a 1989 BMW 325is that I've owned for over 6 years. A couple days ago while warming up the car with the rear defroster on, the whole rear windshield exploded... It didn't projectile explode like you might think, I have tint on the rear and that's probably what kept it from imploding or exploding.

Background to this story is I live 50 miles outside of Los Angeles, CA and the morning of the occurrence was foggy and there was condensation buildup on the whole car. I was using a squeegee to clear off the back window and I noticed when I got to the drivers side after clearing half the window that there was a small spot of bubbling water. I ran in the car and immediately turned off the defroster in hopes that this would stop the bubbling, by the time I went back to the window it was still bubbling and I screamed to my younger brother, that was in the car, to turn it off. He hesitated to my frantic demand and asked, "What?" and no sooner than he asked, the whole window made a sound similar to popping a zipper baggy with your foot. The safety glass did what it was made to do and fractured into millions of pea sized bits but remained attached to the tint on the inside of the rear windshield. When I observed the aftermath of my now destroyed rear windshield, I looked at the spot where I saw the water bubbling and have come to the conclusion that there was no foul play but the fact that water and electricity do not mix. In my case the weatherstripping had a pin-sized hole that caused water to make contact with the heating element that spans across the whole windshield causing it to fracture.

My question is, Has anyone else experienced this personally and to our fearless Mythbusters is this a myth you can take on? Its not a myth for me because I experienced it first hand and I want to know exactly what happened inside that glass that morning that could cause this fairly expensive repair. Please let me know. Thanks- Jon M White

I know someone that had a friend install a new radio for him. Somehow the guy got some wiring crossed and it turned the rear window defroster on whenever the key was on. The guy was driving home from his friend's house that installed the radio when the back window exploded into tiny pieces. The funny part was that the guy though that someone had shot out his back window and sped off the the nearest police station to report he was being shot at. It was the next day when the window was replaced that they discovered the mistake in the wiring.

So I guess the answer is yes, an overheating rear window electric defroster can cause the window to shatter.
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If water were to get into the window near the element, and boil, then the resulting steam could blow the window apart.