I don't know if you know about the search feature in OpenIV, it's not that clear how it works, so a lot of people haven't used it. But if you press CTRL + F3, type in the name of the file you're looking for, like luxor.yft, it will list all of the places it finds that file.
What you are looking for in that list, is the latest version of the files, which is quite often in the highest numbered patchdayXXng folder. Iy can also be in the mp patch folders as well. When you find where that is, replace the files in that location and that should ensure that the changes show up in game.
If they ever release another patch and the car stops appearing, that probably means they have released a newer version and you will need to replace those instead.