
Most car sickness is a symptom of anxiety, rather than true motion sickness. The anxiety may even have started as a puppy, for example, when the pup was transported to his/her new home. It's not always obvious (like you only take the dog in the car to the vet, or the dog really does not like dog shows and that's what getting in the car means.)
For anxiety, gradually associating the car with positive experiences is the cure. Starting with getting the dog into the car, giving a treat and praise - but not going anywhere - graduating to very short drives (five mins max) - to longer drives to somewhere nice (for a run, for example).