We use one with Merc. He wears a harness and then there is a strap that clips into a regular seat belt. It seems to work ok, I haven't needed to break suddenly so I can't tell you how it holds up in an emergency (fingers crossed) but it keeps him in the back seat which is mostly what I need. I have a small car so it allows him to sit with his head poking in-between the two front seats which is probably not ideal, but it doesn't allow him to come all the way through. And he can sit or lie on the back seat. It works for us.
We have the same one for Holley that clips into the seatbelt lock. My seats are shorter thought because I have an SUV so she still tries to come up to see us. They aren't adjustable except for tying a knot in it or sewing it. It seems to be okay but I have not had an emergency situation yet either and we still plan on using her crate for longer trips. We use the harness for the vet and class. Our trainer said that she just needs to get used to it and it will be a very effective tool. I hope so.
I need to get one. I almost bought one at the Orvis store, but I thought better of it at the time. Now, it is a must do! We crate Pumpkin for longer trips or taking her to put on birds away from home; however, it is the smaller car trips that have become unsafe. She climbs all over the place and once got wedged between the 2nd & 3rd row seat. She has slide off the seat several times when braking. At least when she was wedged, she remained in one place until I could stop. Pumpkin can curl herself into a ball, stilt legs and all, so she fits in my son's Graco, backless booster. Remarkable. Anyway, it's off to get a seat belt harness and pet corrector can for jumping :-\
I stopped cleaning the nose and tongue prints off the car window! Only because I didnt have the time though, not because the seat belt stopped him smearing his face on it . Unless you have a really wide car.... But for Merc to be able to sit up and not be tied in the down position the belt is long enough that he canstill reach the window.
I am actually hoping to get a new car soon, something of the hatchback variety. Then I can crate him in the back or have him confined to the back in some way. Right now he rides in the front seat and I pray he behaves. He has not been on any long trips though, I would most definitely have to do something else for that.
I put Copper in the back of the truck now. When he made the transition from rear seat to truck bed I made sure to put him on his _ _ _ a few times. We did some very quick turns and stops so he understands what can happen. He very rarely has any problem and is usually sitting down for fear of being thrown around. Even when he is on the seat which is seldom, he knows he must sit. ;D