Alright so, I'm going for the British setting in GTA V, and I downloaded everything I needed. But before I had all of this, I remembered that I had some custom Police cars already installed, so thinking that when I installed the Package using Package installer in OpenIV, I thought it was going to overwrite the previous cars I had installed. Buuuuut... Nope. So I launch the game and check to see if it worked, it only worked with the cars that I hadn't modified. (Ex: Riot Van, Bike, Undercover Granger/Buffalo) But the rest of the cars I DID modify were still the same from before. 

 

I was an idiot and never created a mod file and just installed the car skins as is. 

 

What can I do to remove these cars and have them be replaced with the British Police Pack I used?

Nothing, you've to restart on a fresh install, and since you didn't use a mods folder, well, have fun redownloading the whole thing :)

Also, using the search bar to avoid making mistakes like this is not an option, my friend :)