I was watching a show not too long ago, and the man said that it could be because the dog's had bad experiences in the car. The dog gets in the car, goes to this strange place, gets a needle then goes back home...so of course, the dog's gonna think that the car's a bad thing. He solved the problem by having the owner feed their dog in the car at everymeal then spending a few minutes in there every day...then gradually working up to driving around the block.