A exploit that people use is to crouch or duck while driving to make it so people can't shoot you.

A way to "fix" this would be to make it so your avatar can't move it's torso in certain parts of vehicles but people can actually duck while driving so what I'd like to suggest would be to make it not an exploit but a feature by making it so if you're character crouches or leans forward to far it narrows your field of view by darkening the screen.