Bonjour Lesley !
In practice, "Je suis monté en voiture." and "Je suis monté dans la voiture." mean the same thing.
The nuance here is that "monter en ..." is more general, literally "to get in car", so it's used when the context is clear which car you're getting on.
Any time you need to be more specific (i.e. I'm getting in your car.), you will use "monter dans [ta/leur/cette...]...".
I hope that's helpful!
À bientôt !
