TeamVanilla In the shipped game each level draws its traffic from a short list of its own. VehicleRandomizer gives all seven levels the same roster, so any of its cars can turn up anywhere.
How it does it
Each level is set up by two scripts. When the game reads them, VehicleRandomizer hands it a copy with lines added: every roster car joins every traffic group in one script, and its model is loaded ahead of time in the other, which the game needs before it can spawn the car. The game's own script reader does the rest. The files on disk are only ever read.
The roster
13 vehicles, each one tested in the game on its own before it was allowed in. The roster is built into the mod rather than read from its config file, so a car that once caused trouble can't come back through an old setting.