18 month old refuses to sit in carseat or stroller?

Of course. All kids refuse to do something at sometime and will throw a holy fit to get their way. Get ready to start deciding what rules will apply. There is no negotiation with it comes to some things like car seats. I don't bother even worrying about it. I say, no, sorry, we need to be in a carseat to be safe; and then wrestle him into it. I don't care about the loud screams - I've heard louder and you will too.