When I joined the Rust team as a programmer last year I was tasked with finally making the old modular car concepts a reality, and along with some dedicated work from Thai (3D models), Alex Rehberg (audio), Petur (FX), Howie (icons) and others that's mostly what I've been doing since.
Currently modular cars can be found around the map near roads, with a semi-random selection of modules attached. They come in three chassis varieties; essentially short, medium, and long.
Minicopters, which used to follow much the same spawn system, have been temporarily disabled. They'll be coming back with a new way to acquire them.
Getting Your Car Going
Cars spawn with heavy damage, no fuel, and no engine parts, and won't run in that state. Low-tier engine parts can be found around the map, and higher tier engine parts can be purchased at the Compound (Outpost), and then also researched and crafted.
Higher tier parts make your vehicle faster or more fuel efficient, but all will make it run. Look at the engine on your vehicle (or you could even have multiple engines) and interact with it to see its internal parts, then fill every slot with the appropriate (and not broken) component to make it run. You'll also need to add some Low Grade Fuel to the tank in the back.
You can repair each module on your vehicle with the hammer or at a vehicle lift. If every module on your vehicle reaches zero health, the vehicle becomes a wreck which can no longer be used, before eventually being destroyed and spawning as a new vehicle elsewhere in the world.
You can buy a vehicle lift at the Compound, which is also researchable and craftable. This will let you edit vehicles in your base or wherever you place it, moving modules around or swapping them in or out. A vehicle requires at least an engine and a driver's seat to be allowed to leave the lift. The lift needs 20 electricity to run.
You can't craft vehicle modules or a new chassis - those need to be found around the world, to make sure things don't get too crazy performance-wise for the server. But you can swap modules between vehicles or players - they become an item when removed.
Module placement affects the weight and centre of mass of the vehicle as well as whatever feature that module provides. Engines do the obvious, seating adds mount points (and yes, you can
have multiple driver's seats), storage adds a container for items, the flatbed can be stood on while someone else drives, and the big tank holds a large amount of water. Not fuel, unfortunately, because Rust doesn't consider fuel a proper liquid (yet).
Console Commands For Admins
Use "spawn 2module_car", "spawn 3module_car", or "spawn 4module_car" to spawn a car with a semi-random selection of modules attached.
"spawn modularcarlift.deployed" spawns a vehicle lift.
"modularcar.outsidedecayminutes" sets decay time, which is 216 minutes by default. Interior decay time is currently 10x the outside time.