I had no idea this was even a problem, until this happened to me today. I recently purchased a used 2006 Toyota 4Runner Sport Edition. It was a chilly, rainy afternoon & I had the heater going in the front & decided to turn on the rear defroster, I started smelling a burning smell, (I originally thought it was some vehicle in front of me), but the smell kept getting stronger, then out of nowhere my entire back window EXPLODED glass all over the back cargo area, scared the shit out of me! I immedittly pulled off the highway to gather my wits and access the situation, I noticed that there was some cracking sounds still happening after the glass shattered. This is gonna cost me at least $200 to get a new rear window. Coincidently I have a 2007 Toyota Solara with a rear window defroster & I have used it multiple time without any incident. I'm just glad no-one was hurt.
When I do get the glass replaced I will NOT be connecting the rear defroster to prevent this from happening again.