I know that for example in the vanilia Panto sometimes appear custom parts in traffic.
The file "vehiclemodelsets.meta" is responsible for this, but it always crash game when I tried to edit.
The best should be mod script with editable values in Notepad... Maybe someone made such script? Idk.
You can use my mods from Menyoo, of course, but only you can drive, no in traffic...