My lil guy used to get car sick too. It was so bad he'd be vomiting within minutes after we left the house, even when he hadn't eaten for hours before we left! A woman at Jake's Dog House recommended an elevated car seat/bed. She said it works because the foam structure, which is about 24 inches thick, absorbs the motion and its the motion that makes the dog sick. The car seat/bed sits on the back seat and is held in by the seat belt. It's covered in sheepskin that is removable and washable and it has built in compartments under it for storage. I attach the buckle to his harness and he's so high up that he can see out the window. At first I thought it was a bit ridiculous but I had tried just about everything else so I bought it. He hasn't been car sick since and now he brings me his leash and harness to go by byes!
It's the best investment I've made for him so far, I just wish I would've found out about it sooner.