My Macy used to get sick even during very short rides. My Vet said to withhold food and give Dramamine, which did work. The problem was the short rides such as to the Vet or Groomer.
One day while I was walking around a Pet Store, I found a cover for the back seat that attaches to the rear headrests and also the front headrests. It lays on the seat and has a little bit of padding. It kind of reminds me of a hammock.
Ever since I started using this not only has the car sickness stopped but she is way more relaxed. Before she would only lay down for short periods of time, now she's out cold and snoring. :-)