Congratulations, you've entered the terrible twos. What a time to be alive! They're only slightly pipped, in my opinion, by the three-nage years but who am I to judge?
Now, you have something to celebrate here, your daughter is learning she's an independent agent in the world, with the ability to say yes and no to things that suit, or don't suit, her. We want to encourage that behaviour, as it's the same skill-set that prevents her from being bullied or pushed around or peer-pressured into things. So, if you look at it like that, what an amazing toolkit she has.
But, and this is a big but, she needs to have her nappy changed and, it's winter so getting dressed is also a must. The car seat is also a non-negotiable. So, here are my gentle tips:
1. With the clothes, can you give her a choice between outfits? Sometimes a Clayton's choice is enough to get a child to have some feeling of control while the ball remains firmly in your court.
2. I set limits about the nappies because it's super gross if you don't. These are limits about which we can't negotiate. You need to say, "having your nappy changed is for hygiene, I can't leave the nappy on you". And, if she still resists, you can say, "would you like your nappy changed in the bedroom or the bathroom?" Again, you've given a Clayton's choice and that may be enough. Otherwise, you'll have to set the limit. If you have to force her, because it's an actual limit, she will cry, and that's okay. Crying is her processing her feelings. But, you're right, this is a limit that is pretty gross, and has medical consequences, if you don't meet it.
3. I'd follow the same process on the car-seat, obviously substituting sitting in the car-seat for nappy changed. Again, crying is okay, and will probably be the outcome, but that's okay.
There are a few things I'd say about this, the first is that this behaviour is a good sign, even though it doesn't feel like it.
Second, where you can be flexible be flexible, but some things are limits and you need to hold the line on those. There's no shame in holding a limit, and also no shame in her being sad or angry about that. I get sad and angry if I get a speeding ticket, but the police officer isn't swayed by my 42-year-old tears, are they (sadly, my bitter experience is they are not swayed even in the slightest)?
Third, I'd implore you to say no as nicely as you say yes, you can be kind while setting the limit and clearly, and briefly (very briefly) explain your position.