Since we're getting ready to travel in minutes...

I just have a regular dog harness. You can slip the seatbelt between the dog and the back strap and buckle them in. That's what I do. They can easily lie down if they want to.

They also have small leads that strap into your seat belt and hook onto the collar. So it will keep your dog in the back seat, but they can still move around back there if they want.