I had the same problem with Cosmo. I read some great advice and it has worked. He now only gets the jitters once in a great while, like a trip subsequent to a vet visit.
I used to take him out the car and just sit with it running while listening to the radio for 15-20 minutes. Did that for about a week + then would take small trips around the block, window down music on, then back home. I did that for a week +. Then there is a park about 4 miles from the house for about 2 weeks we would drive there twice a week, he would get to run around and play and then home.
He got used to the idea that everytime we went in the car it wasn't to go to the vet or groomer which is frequent in the first year.
He still does not LOVE the car like my friend's yorkie but he doesn't jitter or vomit. Good luck, I hope this helps!
