Hopefully you'll bolt them down securely so they don't be come lethal projectiles when you have a wreck or stop suddenly. IMHO Leave them at home where they belong - they're made for filling a 10x20' room not a 4x6' area.
Back in 1990 on summer break, we put one of my Pioneer CS-607 speakers in the back of my roommate's Supra.
We rerouted the rear speaker wires to this thing in the hatchback. We were running them directly from a Blaupunkt head unit. It sounded pretty good for a couple of college kids that had nothing better to do with those speakers for 3 months, but I think it did damage to the head unit in the end. The speakers still kick butt out in the garage.