I have something similar to this one:
http://www.petautosafety.com/padded-dog-car-harness
The harness goes with the dog and there is a short (~4 in) belt that clips to the harness with a carabiner but stays permanently attached to the seat belt with a locking clip - super easy to install and keeps my pup in the back seat. He can stand, sit or lay down comfortably
PetSmart sells a couple of models.