The really serious SPL competitors actually install a chain with a turnbuckle between their roof and their floorboards in order to keep their glass in place.

Now, I've never been to one of these competitions, and although I can understand the "more is better" attitude...

WHY?

What possible reason would one have for making a car sound system that is so loud that it could physically injure you if you're in the car with it?

I understand that if a sound system can play loud without distortion or coloration, then it has that much more headroom to play cleanly at normal volumes. And I understand that one might want to open the back hatch and play a little music at a picnic or whatever. But you're talking about the kind of sound pressure levels that come out of the PA at a rock concert. This scares me, and I've been to some loud rock concerts in my time.

Not to mention that such a car would be a really huge theft target...

Hey, now THAT's a good reason... You could wire the burglar alarm so that it fires up Nine Inch Nails at maximum volume if someone breaks in. If the thief manages to steal the stereo anyway, he wouldn't be able to listen to it because his eardrums would be vaporized.

Course, woe is the owner who accidentally sets off his own alarm while he's in the car...