A long time ago we had a dog that vomited every time he was put in the car. (What was amusing is that he used to lay in the seat on a pillow between the driver and front seat passenger. Whenever he felt the nausea coming on, he'd turn toward the driver and puke down his leg....
Guess you had to be there...and not be the driver).
Anyway, we asked our then vet about this, and he said the puppies are like children. To avoid the car sickness/motion sickness, one must never ever let them look out the side windows of the care (makes them very aware of the scenery woshing by). He said we should hold the dog upright and help him to look out the windshield of the car.
Either this worked or ol' Percy outgrew the need to heave. Give it a try.