Ein had
this one, for anyone who is shopping for a car harness. We chose this particular one because we thought the fleece padding would make it more comfortable. It doesn't have any space for the dog to move, though, so we also bought a little strap thingy from Petsmart that is used to hook regular dog harnesses to a seatbelt. I figured with that long of a drive, he ought to at least be able to lay down.

I have no clue what the other thing is called, but it's basically just a strap that has a loop on one end (for the seatbelt to go through) and a hook at the other end to attach to the harness. It looks kinda like
this , but it's not that exact one.