Try going for short rides - just around the block and gradually increase the distance.
Bach Remedy Scleranthus - I had one dog that used to get sick - I dosed her with this and she didn't get sick at all. Used it a few times, and gradually she was fine riding in the car without it.
I have two that got car sick every time you started the engine. When they were pups, I'd hold them on my lap and DH would take us on short trips, gradually increasing distance.
I crate my dogs, as I feel it's safer for the dogs if something happens.
I really don't remember if I used the scleranthus on the puppies or not. But the going for short rides got them over their issues. They ride crated now with no issues. THey've been up and down the east coast several times.