I had some experiences with my three boys which led me to this book called "1-2-3 Magic" by Phelan.
You might find some hints in this book to help you. What I would suggest is some de-programming. I mean, no DVD, no favorite seating, no pillows even: when there is any fussing.
To get there, you will need to decide what to battle about first. Say, no raised voices. That means, for a while, you will to refrain from being "verbally repetitive" about shoving, entertainment choices, etc, but when there is a raised voice the count begins. They would have consequences for getting to the count of 3, and I would be lenient with age, where I would get to 3 quicker if it is the oldest but warn the littlest that you would count if he continues. (You really have to go with your motherly instincts.) Anyway, their consequences could be a loss of privilege (like TV time at home, or computer games at home). The consequence may be that you pull over and wait for five minutes, asking for silence.
I don't know how to describe how this 1-2-3 counting worked for me. When I used this 1-2-3 counting method, I really had to work hard on keeping my emotional response from being an angry and very frustrated response. LOL!
In your situation, sounds to me, you have a kid, or two, who may not be good with taking turns yet. Then, you have a kid or two who should obey more readily.
Consistency was my other obstacle. Say up front what you are going to do. "Disturbing the driver" can be dangerous on any road!
I'm sure all of the responders are going to wonder what works for you!
Good luck!