it's only so durable.
make sure it stays ventilated in the car so it won't overheat
do not leave it in the car in cold weather and then fire it up to play
do not leave it in the car in hot weather, ever.
best practice is to always carry it with you in a nice padded bag when you leave he car... unless the weather is very moderate
EDIT: and i'm sure it's possible to ruin a DVD if the disc is spinning and you hit a good enough bump.