So i first tried to install the car with Open IV and was doing it in the main folder when i started the game it redownloaded files deleting them. So i read to do it in the copy to mods folder which i did and now when i get into game it is still the default car. Is there another step i have to do such as make gta use the mods folder rather than default or something like that? thank you for your help!

The file structure in V is unlike any other GTA game, the cars are in different folders completely

The main police vehicles are inside x64e.rpf/levels/gta5/vehicles.rpf

whilst Police3 (The Interceptor) is inside update/x64/dlcpacks/patchday4ng/dlc.rf/x64/levels/gta5/vehicles.rpf (i think)

so i just have to make sure that it is in all of the files mentioned in the mods folder? Thank you

It depends on which car you are trying to replace. Models for different cars are located in different folders. Read the installation notes on the mods you download.