Whenever mine ride in the car with us, they both ride in a large wire crate together. It has lots of blankets and stuff in it and it sits up high so they can see out. When I just had Hobbs, I would take him everywhere with me. I would just let him roam the car. He usually sat on my shoulder.
But I realized that it was just too dangerous for him to be doing that. Now that we have Hootie, I don't feel bad about leaving Hobbs at home anymore. I don't take them with me on just little trips around town. They only go if we are going out of town or something. So, there is no reason for them to ride in the car outside their crate. Hootie actually freaks out if she is riding in the car and is not in her crate.