The "monster sub box" blocking the vent ports in the rear bumper have way more to do with this than air pressure gradient from heat or driving at speed.

 

Yea I'm thinking it all added up. Seal the trunk, turn the music up, turn the heat up, hot air rises/gathered to that point= boom. Probably was under stress. Hope you've opened that trunk up. I know this is old but anyone know how much would this of cost?