Mine have car seats in the back seat. Loki loves his and goes right to sleep. Sammy didn't like hers at first, but after 2-3 rides she realized she was not getting out and she stopped barking at us. She rides pleasantly in hers but she doesn't sleep unless she's tired, she prefers to watch everything that is going on.
We have Snoozers too. There are lots of different styles. You can get them online or check with your local pet boutique to see if they have one. They are anywhere from $75-$200 but trust me they are worth it!