Consistency--practice the exit process the same way every single time. My husband is a practitioner of the commonly used Erratic Training Method; consequently I still deal with this problem from time to time....grrrrr....
You could also treat your dog while he is sitting and waiting. That reinforces for him that that is the correct behavior. If you treat after he moves and gets out of the car, you've just taught him the wrong thing.
That's my best shot at it. Good luck!
Oh, with the treat on the ground thing--along the same lines--I will actually throw the food around and the dog is expected to stay--but I always make sure I then pick up the food and give it to him while he is staying. I don't release the dog to get up and get the food for himself. Again, that emphasizes to him that he is never to go after the food on his own but is to wait until I give it to him.
You could also treat your dog while he is sitting and waiting. That reinforces for him that that is the correct behavior. If you treat after he moves and gets out of the car, you've just taught him the wrong thing.
That's my best shot at it. Good luck!
Oh, with the treat on the ground thing--along the same lines--I will actually throw the food around and the dog is expected to stay--but I always make sure I then pick up the food and give it to him while he is staying. I don't release the dog to get up and get the food for himself. Again, that emphasizes to him that he is never to go after the food on his own but is to wait until I give it to him.