P.M.
I think you're doing the best thing. Pull over every time, and plan extra time into your trips until he's got the message.
I'm not sure what you mean by making it a "big issue." Heavy discipline? Talking it through?
If it's punishment that makes him more defiant, try emotion coaching or empathetic parenting. You can google these terms for instant gratification, or get the practical book How to Talk So Kids Will Listen, and Listen So Kids Will Talk. These techniques work, and you'll reach for this book often.
This approach teaches you how to give your child an opportunity to apply his own understanding to find appropriate solutions (yes, kids can do this, and will, when given the chance!). And it's NOT lenient parenting – you can also learn how to present your own needs and limits most effectively.
Good luck and happy parenting.