First off, you'll need a gameconfig mod to get mods working. I recommend using the gameconfig linked below along with its prerequisites for the Legacy version of GTA 5.

https://www.gta5-mods.com/misc/gta-5-gameconfig-300-cars

Next up, this is the GTA IV car pack that I used to set up this modification. You have to install it. It comes with an OIV, so it will install automatically for you. Just run the OIV installer in OpenIV.

https://www.gta5-mods.com/vehicles/ivpack-gtaiv-vehicles-in-gtav