L.
Well, I'm sorry to say they don't get any better or more co-operative when they get older.
I would arrange to be taking them somewhere they want to go, then, when they are not in their seats, simply pull off the road, turn the car off and tell them you are sorry but you cannot go unless they stay in their seats. You might have to be creative and tell them if they are good on three trips, they will get a reward or some such thing.
Mine are older and constantly either fight or do the silly poking at one another thing. I have a 17 year old who constantly screams at the two younger kids. They are real close in age (11 & 12) so I get the twin thing.
I would suggest "Parenting with Love and Logic" the book has been a great help for me.
If you can't find it in a bookstore, try loveandlogic.com or something like that.
It's important to remember that kids learn by example. If you remain calm and empathetic, you will be much more effective than if you get upset by their misbehavior. Wish I could remember to do that myself LOL!
Good luck, and remember; in 13 years they will want to drive themselves!
L.