Shot gun trouble...

Recently the 11 year old and the 13 year old have been fighting over who gets "shot gun" for the front passenger seat in the car. They go over the "rules" of shot gun, 'must be outdoors heading to the car', blah, blah, blah...who cares about the rest. I don't care who sits there. I care about peace and not having problems over a seating position. I want them each responsible for their own behavior. After playing referee more times than I should, even setting consequences for fighting behaviors with each instance and no shot gun for that trip and still having problems, I announced that the next problem with it that I had to become involved in would cost them both the shot gun seat for a month. Of course we had another problem. So now they have a month where if both of them are riding in the car, neither gets shot gun. They have a month to work out a rotation schedule with which they will inform me what they have decided and manage it on their own, no fighting. They wanted me to decide per car ride which one's behavior had been the best and who deserved to sit there. Wrong. That will of course invite more arguing and proclamations of unfairness and leaves them not responsible for their behavior. So,I've heard their discussions of daily, weekly, monthly rotations. Anyways, we'll see what they come up with, but it has to be monitored by them, kept up with by them. Next problem will invite another month of no shotgun at all whether or not they are in the car together, so it is in their best interest to figure it out.