I drive a smart car so I wanted to transition him to the passenger seat of my car and make sure that he would be safe there. The first thing I did was purchase a dog car seat. I hooked it up to my passenger seat and it was raised so that he could easily look out the window while being strapped into the car seat with his harness.
The only time he really disliked it was when I got out of the car without letting him out first. I loved it because I was able to keep my purse and other things on the seat under him without it being an issue.
If I recall correctly, it was supposed to hold up to 20 or 30 pounds, which it didn't do. Ares has weighed 17 pounds steadily but the seat started ripping in one of the corners from the weight. Overall, I really liked it and it did a great job of keeping him safe. One day when I was driving, the car in front of me slammed on their brakes and while Ares flipped right out of the car seat, the fact that he was strapped in prevented him from going straight into the dashboard or windshield. It scared the bejeesus out of both of us but definitely served its purpose of keeping him safe.
My next problem was that it wouldn't fit both Ares and Luna and because I have a two seater, I needed a way to safely transport both of them in one seat. To clarify quickly, the "trunk" of my car is so small, I have to collapse their kennels in order to transport them so that option, while the safest by far, is out of the question.
I ended up taking the carabiner off of the carseat and off of the car seat harness that Ares never used and hooked them to the buckled seat belt. This way, I can use their harnesses to attach them to the seat belt via the carabiner. (They have really awesome harnesses that allow me to do this and still give them plenty of room to move around and even jump up to stick their heads out the window if they wish.)
After I did this, they still jumped in my lap every chance they had and Luna would sit in the passenger seat and just whine at me the entire time. One day I happened to have their large dog bed in my car so I put it on the seat and they were in heaven! Now they get in the car and sit and sleep in their bed during car rides. They'd still much rather be in my lap but they don't spend all of their time trying to wiggle their way into my lap anymore.
I don't know if they harmony in the car will continue as Luna grows, but for the foreseeable future, this is really working for us.