I was wondering, are weapon shots stopped by vehicle part completely? I tried to shoot from driver’s seat of a car through open door and shots never came out, I managed to kill Z only when he was adjacent to me (on door tile).
some but not all parts will stop bullets.
I think it’s anything that would push something while the vehicle is moving (I think it includes quarter panels which you should be able to shoot over :-/ ) I know Boards, windows, and the various doors will block if they’re closed.
The most annoying are mirrors 
they also stop everything from arrow to machinegun bullets and probably even 40mm grenades.
I don’t think I’ve ever had a mirror survive driving long enough to realize that they were bullet shields! I got so sick of them breaking off every time you nick the slightest thing that I gave up and switched to security cams.