The vehicle system is a fairly powerful and nifty thing… that is very poorly documented. A few basic points (most of which I figured out from trial and error, so some of them might be wrong!):
-“slot”/location - some things require a specific layer/location/slot (thus making other things impossible on that square which use that same “slot”), and some don’t (always can be installed, once per square). “slots” that I can figure out are fuel (gas tank of any size, diesel tank of size, water tank of any kind or size, napalm tank, storage battery of any size), engine (any kind - only one per square - this includes foot pedals), roof (things that go ON the roof, whether you have one or not - funnel, solar panel, mounted weapon, etc), armor (all the platings, plus some other odds and ends from parts packs, such as the rebar reinforcement and the shock absorber), and possibly more… it gets a little harder to figure out how some things interact
-storage - in addition to “slots”, you can only have ONE part per square that gives you storage space. Most mounted weapons give you some storage space, so they can’t go on a regular seat (hence the military seat that has no storage). I suspect there are some things that count as giving storage, even though they don’t (trunk door or sliding door, for instance).
-“foldable” - if every part you install on a vehicle is “foldable” (has a folded volume), the vehicle can be folded. In the stable build, it also requires controls. Currently, you must be able to PICK UP the folded vehicle to unfold it, so be careful about the weight!
-“boardable” - spaces that have something “boardable” will not hamper movement when you cross them
-“opaque” - can’t see through them.
-regular vehicle controls have a “dome light” - the light comes from the square with the controls (when you turn it on). Other kinds of controls don’t have that at all. (Yes, you can install multiple sets of controls on the same vehicle.)
Storage batteries are considered “fuel”, but other vehicle batteries (motorbike,car,truck) are in the “free” category, so you can put one of those on every square of the vehicle, if you want to (the outside edges are not recommended).
I’m sure other people around here can tell you some other important things, but that’s what comes to mind right now.