As I just redownloaded it again, I had the same issue. Luckily, the solution is very easy. Launch OpenIV and browse to mods/update/update.rpf/common/data/levels/gta5 and drag and drop vehicles.meta to your desktop. Once it's there, right click it and open it with either Notepad or another text editor. Press CTRL+F and type in the car that you edited, which is in this case Sheriff. Scroll a bit down until you find the following line </requiredExtras>. Replace this with <requiredExtras> EXTRA_2</requiredExtras>
And that should do the trick, then save the file again and drop it in the OpenIV folder again. If you don't get something, don't hesitate to ask me