Children this age are striving to be independent. They see that adults have power and they want it. Right now I would say that he has quite a bit of power but since his is 2 he does not know how to use it. (and he probably should not have that much). It is all about choices, you need to offer him choices that are acceptable to you.
For example...
Would you like to wear the blue pants or the brown ones?
What would you like to put on first, your pants or your shirt?"
Would you like to climb into your car seat or should I lift you in?
Would you like to buckle the chest clip of your car seat or should I do it?
If he doesn't choose right away, ask him again, if he still won't choose then choose for him and tell him that you are choosing because he did not.
The key to this is to offer him choices about EVERYTHING (color of cup for juice at lunch, pick up toys while acting like a robot or a puppy) but make sure that the choices are for things that, in the long run, really don't matter. That sounds funny but who cares if he wears blue or brown pants or if he climbed in the car seat or you lifted him in. The point is that you control that he is getting dressed and that he is getting buckled up (the desired behavior) he just feels like he has the control because of the "options" you are giving him.
From reading the other posts it sounds like you are the caregiver, not his parent. It may be more challenging because I am sure you have a different style than his parent does. Chat and see if you can make an action plan for the little guy. Kids want limits at this age. They want to know how they fit into their world but they will test them all. There are a couple of great books How to Make your Children Mind Without Loosing Yours by Kevin Leman and Parenting with Love and Logic by Foster Cline and Jim Fay. Both of these books talk about giving choices and how it works.