Well,
It's hard to know unless your there, sometimes. I will assume that you have done everything I would have done by now and are still not managing to get him to Cooperate.
Why do I say that? Well, there's certain mood you set by EXPECTATION. I am sure you have set the mood already. That is that he doesn't commonly get away with being disobedient. I will assume that he just REALLY likes to do his car seat himself, which will be great when he's good at it.
So, assuming that this is what drives him, what about bringing the car seat into the house in between runs so he can "practice"? You can practice taking turns and create a common language about who's turn it is like it's a game....pretend to buckle it in. Make the buckling part funny? When you take him to the car. show him how hard that buckle can be and struggle against it like you can barely manage it yourself while talking to the buckle? I know it sounds crazy.
Like "OK, watch this, I can buckle it! EERRR UUUHHH EEERR OOOHHH OOOOO uuugg.....eerrr" He's laughing at you because you can't do it. A few faces would be good here. If you're funny, he'll want you to do it all the time.....maybe.
You know, you make a game of it when you're not in a crunch.
I may be way off base here.
Then you are in a hurry. He wants your funny struggle, you don't have time....he anxiously gets in the seat anticipating his show....you snap it in quickly and act amazed! "WHAT! Did you see that! Oh Man! ...Hey buckle, are you awake? Hey buckle...answer me!" Your'e driving the car well on your way and still acting amazed that the buckle was asleep.....or maybe it's sic...hey buckle, you OK?
My son thought it was funny when I talked to the road. Now you're on your way and your talking to the seat.
It wouldn't last forever. He would get out of the habit of the struggle.
Again, I could be wrong. If I have a real problem, I try humor. That's just me.....unless of course it's one of those days when laser beams are shooting out of my eyeballs. Then it's totalitarian rule.....usually not pretty.