I'm curious about the OP's statement that he's sure it was the original, factory-installed window.
If I had a window crack, and attributed it to "thermal stress" I'd be trying to understand why this happened. It shouldn't. Perhaps the window frame is a bit wonky, and the window didn't fit perfectly. Then the movement of constant towing may have caused a crack to develop. And maybe this had happened once before and what you're dealing with is a replacement window.
But you said you were sure it was factory. So. Oh well.
Still there must be a reason it cracked. This doesn't seem normal to me.
-Speak
