The title is a little vague but basically you can have one of several parts as a “wall” on a vehicle:
[ul][li]Welding Rig, RV Kitchen, Mini-Fridge - All are “Solid” but also transparent[/li]
[li]Board - Solid and Opaque[/li]
[li]Quarter Panel - Solid and transparent[/li]
[li]Steel, Alloy, Spiked, Hard Platings - Passable and Transparent[/li][/ul]
Any of those can be installed as a “wall” part, and as a result armor cannot be installed anymore if you want that section impassable. (For example bolting Steel plating to an RV.)
On a semi-related note, what is “Quarter Panel” ? Other than a transparent board.