
My girl travels on the seats, she doesn't have a special car harness just her normal one. I have the lead long enough to allow her to stand, sit, lay down and turn around but not long enough for her to get off the seat.
If using a dog seat belt that has a clip to clip it in where the people seatbelt goes be carefully it don't get jammed in. I got a universal one from Wilkinson and clipped it in and it got stuck in my mother's car, her mechanic couldn't get it out and said she would need a new one. Luckily at a service at the dealers they managed to get it out.
I now have an old lead tied around the head rest that I clip to her harness, which I find better as with the seatbelt one she gets her legs tangled in it where as now the lead comes from above she doesnt.