One of my dogs was terrified of car trips when I got her and I had a big road trip planned 6 weeks from when she came into our home. The minute she was put in a car she'd start shaking and drooling, totally stressed. Then at some point in the trip she'd almost always throw up. But I had this vacation planned in 6 weeks and I had to get her acclimated somehow.
I started by taking her on almost every quick little errand I have to do during the week, like going to the post office, or the bank. We'd also go to the local dog park frequently, more for the drive than the park since my dogs aren't really into the dog park (it's like a bad doggie singles bar with a lot of jerk dogs...and jerk owners
The drives were short so she would never get to the point of throwing up on them and it allowed me to work on getting her calm and in the car, letting her get used to her harness, and giving her just enough of a ride for her to start getting used to the whole experience and sensation without it overwhelming her.
I also put her bed in the car, the one she sleeps in every night. Having something familiar to rest on really helped her get over the fear of the car and helped her stay calm.
I did this for about 5 weeks and by that time the short trips had become routine for her and she did well on them. So I decided to take a small road trip to raise the bar and see how she'd do. It was about 250 miles round trip and I made several "potty stops" for her. She did great! Super calm and no drooling or throwing up. She ate normally before the trip since at the time the dogs were free-fed kibble (they are on PMR now).
On the 6th week the big road trip was finally here. It's about 700 miles one-way and would be the ultimate test for her. By this time, she would actually get into the car by herself and she would sit quietly as I put her harness on. So the morning of the trip, we got up before sunrise, I put their dogs beds in the back seat, they hopped in the car, I got them strapped in, and we were off. I decided to leave before sunrise since I figured the dogs would probably sleep easier on the first leg of the trip if it was dark.
It was like she had been doing it all her life, no problems, no anxiety, no stress. She curled up in her bed and went to sleep until we made the first potty stop about 120 miles into the trip. When we stopped, they got out and did their business, had a drink of water, and we were off.
And so it was for the rest of the trip, and every one since then.
I think the key with a dog that has difficulty riding in a car is to give them the chance to get used to it slowly and frequently using short trips as often as possible. After awhile they will become acclimated and riding in a car will just become a part of their normal life.