if i put boxes in the trunk of my 91' mustang gt hatch will the rear window shatter or crack if i get too loud.

 

No...and...not even close..reguardless of what you have.

 

Only if you build the box too tall and slam the hatch window down on it

I speak from experience - only it was a keg of beer, not my speaker box [:@]

 

I put my box in wrong in me trunk of my conv. I put the top down and it just shattered. Didn't even relized what happend untill it was to late. Ended up costing me about $1,600.

 

if you have subs that will shatter your hatch, I bow to you.

 

No, even if you had really loud drivers it still wouldn't be able to crack/break a window. Theres two ways to break windows in a car with speakers, one would be to play the natural frequency of the glass at a very high volume. The other is if the car is sealed up enough and your speakers are able to produce the amount of pressure needed yes you could easily break a window. However in order to produce that level of pressure you would be looking into welding many areas, sealing your ventalation system, basically making your car airtight. If you've ever noticed a real SPL car( No im not talking about the suv with 15 sony subs in it) Im talking the real champs the vans with 2 monsterous subs and have plexiglass windows, that actually need it. those guys can break windows. We're talking 170+ db (125,000 watt systems)just to get an idea a rocket launch is about 150 db, something you can actaully compare to is a top fuel dragster at launch is about 145 db.
So no, not without sealing your entire car and getting drivers with some serious power, you'll be fine.