Go to vehicles/common and find folder with license plates. Copy folder from one of levels and change the name of map with your map name. My mod, Dry Rock Island license plates, could help you.
First find the folder with licenseplates: ...and open it. Here change the selected folder's name with your map name. Open that folder, and open .jbeam. Change the selected lines with your map name/licenseplate picture name_d.png And after change the .jbeam name with ''licensePlate-default'. In .json folder change the same stuff as in the .jbeam. Mine plate mod, Dry Rock Island Plates can help you. Its link is in the post above.