Hi Kim, I am so glad you are investigating car restraints. We travel the highways a great deal in the summer and have had a couple of close calls including one on Monday where our restraint has held 80lb Gavin in place and safe. Kurgo is a brand that a lot of people have had success with. You can search this site as there are a few discussions and pictures on the topic. I do not use a traditional system simply because Gavin is very deep chested and long bodied and I could not find any that fit him properly and fit is key. I use a well fitting regular harness and a coupler which I loop around the shoulder belt in the back seat. I will get a current picture of my set up and post here later.
Okay, Gavin is pictured lying forward with the back seat of my SUV laying flat. His harness has wide straps and is somewhat padded. The strap that goes around the shoulder belt is a coupler, traditionally used to walk two dogs at once. Hope this helps.