I started with my former cat when she was just a kitten. That way she never knew if she was going to the vet, or just for a ride. I got lots of looks from truckers and such on my trips home from college with a kitten on my dash! I just made a small litterbox that fit inside the cat carrier and she was good. In fact, she drove with me and my dad cross country when I moved from Alabama to Washington State! The hardest thing about that was sneaking her into hotels at night.