The only reason I know is because I had my old truck broken into because it had a system in it and the took it. The paint on that truck was flawless and the passengers door was F'd up becuase of the glass falling out.
Well....I wanna say that all the videos I've seen where people blow out their glass with their systems have them facing directly at the hatch. Kinda makes sense when you're putting most of the resonance into the hatch.
My old 15's was hitting 141.5db facing up, had them in there for 3 years, and I've never worried once about my hatch glass.
Build a new box so they're angled, the sound waves will stay in the car and less resonance will be blasted into the hatch. Just my 2 cents. Wanna save you the worry of getting more glass.
You can overpower your subs if you know what you're doing. It's just like you can throw a raw egg and not break it if you know what you're doing. I've over powered every sub I've owned, and only ever destroyed one (kinda happened because it was way over powered, but I just wanted to see what it could take. lmao) It's a nice story, and I may just open a thread about it. I've been itching to tell it lately, as my car might have burned down.
People just don't understand that when the sub no longer produces a nice bass note, and you hear grinding from it that you're not making more bass, but destroying that 200 dollars you invested.
damn....I wouldn't think they would cover glass, since if the rear window breaks most likely it's because of someone breaking in or something hitting it! Not a manufacturer default. That's pretty sweet. Did they give you a hard time about it?