I tried to be heavy-handed about the car seat fights, but it made both of us miserable. What worked for me was keeping a stamper and ink pad in the car -- and giving him a stamp on the hand when he cooperated.
At first, you may have to give her a stamp after a fight, just to introduce her to the concept. Get out the ink pad and tell her, "Look at you! You're buckled in your car seat! Yay -- you get a stamp!" You might have to do this a few times. After a couple of times, start telling her *immediately before* you walk out the door, "Remember -- if you get in your car seat right away, you'll get a stamp!" At that point, if she fights you -- no stamp. And you'll need to tell her calmly, "Oh well. Maybe next time you can get in the car seat and get your stamp. Let's try again next time." She will learn that cooperating means a stamp -- and she'll be happy to cooperate. She'll start to look forward to getting the stamp.
Positive reinforcement can be really effective! This will also work with getting the shoes on and putting the jacket on.