Instead of a seatbelt harness I have Premier (the company that makes Gentle Leaders) harnesses. They have two buckles at the shoulders, so the dog doesn't have to "step in" to them, and they have adjustments on the chest straps as well as around the girth, so you can make them fit right.
Then I use a nylon "tab" ( a leash that's just a loop and a clip, Petsmart has them) I put the tab on the harness, and the seatbelt through the tab and buckle it, then cinch it down to where it's snug when the dog is sitting or lying on the seat. Works just as good as the ones marketed as seatbelt harnesses. I drive a Buick Station Wagon, Earnest rides on the back seat, Phyfe rides in the back on the jump seat.