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If the glass was down when it broke, it was the torsion bar inside the tail gate that acts as a spring to help lift the TG from a open position to a closed one.....it also works the opposite way and takes some of the weight off the TG as your opening it.
If it shattered while it was up, then yes it could have been the rear defogger possibly shorted or something and sent too much voltage to the glass and it caused it to break.
When you do find the exact cause PLEASE post back here what you found.
It hasn't happened to my Bronco yet. But years ago I was driving my father's Jeep Wagoneer, and it also exploded. What a scare it gave me. It happened one more time after that, a few months later (dad was driving it this time). He has since sold it, but I remember reading someplace that there was a recall for the Wagoneers for this issue. Maybe if you look into what the recall was about, it might be something similar to what happened to your Bronco.
Remove the access panel from inside the tailgate. I assume the glass is all in little pieces now. Look inside the tailgate, driver's side. The torsion bends at a right angle then runs out the passenger side of the tailgate. Just above the bend, the bar should be held by some sheet metal wrapped around it. That may have rusted away which released the torsion bar to pop the glass if the glass was down and shatter it. Hint: a camera helps to see where you can't get your head. The bar should not touch the glass when it is down.
well guys i think it was the heater b/c theres no rust or bends in the tail gate the truck is babied and that window was like $500 new so i got a used one (took me 4 mounths to find 1) for $50
i cailed the loacal ford dealer and they confemed the short issue
so let this be a warning for all disconect the rear defroger
My ex-wife and I had a 93 Villager that did the same thing but it was 30 below and the dealer said it was caused by the sudden drop in temp after the defroster shut off. When we had it replaced we opted for the non heated glass.