Could be a power/control struggle; YOU need to get her into her seat so you can get somewhere YOU want to go, and by a particular time.
How can you make her feel as if it's something SHE wants to do?
Find a time when there's someplace SHE really wants to go--create a situation if you need to (like, "Hey, let's go to the playground!")when there's no deadline, you don't need to be there by a particular time, and it doesn't matter if you don't go. When you get to the car, just say, "How fast can you get into your car seat and get it buckled?" (if it's one that she can buckle). If she balks, just say, "We'll go as soon as you're buckled in your seat." If she doesn't, you get out of the car and go back to the house. Then be as boring as possible :) so she'll WANT to get in that car seat so she can go where she wants to go.
The "Show me how fast you can...." worked for my daughter--"Show me how fast you can get in your jammies" (or put on your shoes, or get on your coat, etc." She liked the control and accomplishment.
This is a lot better than a tranquilizer gun (LOL!)
You could also talk about cooperation, how there are times that you do things she wants to do, and times she has to do things you want to do, and that's just the way it is sometimes. Unfortunately for kids, it always seems that they're forced to do all kinds of things and they have no choice or control. Nobody likes that (I'm STILL the kind of person who will be contrary just on principle sometimes!" Whatever works for you & your daughter.
Best of luck.