The car will not penetrate.
The PHB pg.204 talks about areas of effect:
The effect of a spell expands in a straight line from the point of origin.
If there is no unlocked straight line it extends from the point of origin to a
location within the area of effect, that location is not included in
The spell area.
The point of origin is yourself and it is a ten foot hemisphere, then the area that becomes the cabin is of you, yet ten foot radius, limited by car walls
What the spell does not specify is whether this reduced area would also reduce the amount of creatures you can contain in it. As it is magical and not specified, I think it does not reduce internal space. However, that is purely my opinion / interpretation. I would strongly suggest that you ask your DM for a decision about it, since it is quite vague.