Pretty much title, when I install a new piece into my deathmobile it will sometimes change the tile that is shown to the new object, but I like the way the old one looked better. I know about the sha(p)e key, but that only allows me to shape the frame itself, which isn’t the tile that actually appears when you look at the car.
I think they are shown in the order they come in the Examine menu. Meaning Roof section always shows on top when outside the vehicle, and is hidden when inside it. I think it might also depend on which vehicle roof tiles the tile set has.