I had a dog who would always get car sick, until I learned that it was mostly nerves that were causing it. It seemed that if one of us sat in the back seat with him on our lap, he was fine, no sick-up. It was just when he was alone in the back that he got sick. I know it's not a great idea to NOT have them buckled in, but I prefered to hold him for the whole trip rather than clean up puke!
Besides, it was great cuddle time for both of us!