Again, I'd join the forum at
http://www.bestfriends.org/ and inquire about how to prevent dogs from chewing the car.
This works with cats. Felines hate citrus and ammonia odors. They will avoid these scents. Dogs are not the same, but they also smell much better than humans and even cats.
I'm suggesting there may be a scent that would not bother people, but keep cannines away from the car. Another idea is putting a devise that produces high-pitched sounds only dogs can hear and want to avoid. Pets stores may have them.