I'll try to explain this the best I can, so here goes.
Go to your latest patchday folder under the update dlc paths (you should know where to find it). Drop in the model and textures of the car, but have them renamed to police. Then, go your original carvariations and vehicles.meta files, and find the lines for police. Take the sheriff section from bxbugs version, and replace the police lines in your game with the one supplied with his model, and then rename sheriff to police so it works.
If you are still trying to use the openIV auto installer, you need to get inside it to find the files. I use 7zip so all I had to do was rightclick on the installer pack and extract the files, which will open it up to let me see all the files and let me do whatever I want with them
Hope this helps, I totally remember the pain of going through the install for this. Let me know if you have any other issues.
Cheers,
Maxmov