The truth is there does not need to be a consequence or punishment other than your clear unchangeable message. If you make yourself consistently very clear about what you expect, then that’s all you can do. You cannot directly control what your child does. If they challenge what you expect, they are either testing boundaries, or they want to understand your reasoning. So all you can do is be all the more clear and reiterate what you expect and why it’s important for you. And you need to do this always as calmly as possible.
Okay, so you are driving along and your children are out of control fighting in the back of the car, what can you do? First, remember that children may be playing when it looks like they are fighting. In any case, that is their business, what bothers you is the noise and the worry that they might get hurt. Tell them calmly that you cannot drive safely when you are bothered by these things. You may need to stop the car in a safe place to do so and wait until they are ready to oblige. They don’t want to stay in the car any longer than you do.
Do not take your child too seriously, they are not in charge, you are! Watch your own behavior. Defense is engaging in a conflict that would not be one if you did not react to what your child does. Control yourself, not your child. Do not hold your child accountable for your response to how they behave, or you run the risk of them misbehaving to obtain your attention. When you are constantly clear and transparent, your child will attune to your values and expectations more naturally. S/he will feel more secure and less confrontational with you. And if you feel confronted, refrain from fighting back. Consequences and punishment imposed by you are belligerent reactions. When you calmly and clearly state expectations and boundaries, then you are at peace. Whatever you feed will grow.
