I assume this is the right place. I have looked everywhere for a tutorial on how to add a new car file into the game but no avail. I am trying to add a state troopers vehicle but dont want to overwrite any of the existing car files. Can anyone help?

Watch this video tutorial. It should answer all your questions.

https://www.youtube.com/watch?v=-M05f2MYo70

Just in case the video doesn't mention creating a mods folder, you just have to create a new folder called "mods" in your main GTA V folder and copy any .rpf file you want to edit there. Keep in mind inside the mods folder you have to use the exact same file path as in the main GTA folder. For example: 

-x64e.rpf should have the file path "Grand Theft Auto V\mods\x64e.rpf"

-update.rpf should have the file path "Grand Theft Auto V\mods\update\update.rpf"

-dlc.rpf from patchday3ng should have the file path "Grand Theft Auto V\mods\update\x64\dlcpacks\patchday3ng\dlc.rpf"

etc.

Then when you use OpenIV to replace the files, double check that you are replacing the files from the mods folder and not the original game files.