Are the cars addon (DLC) or replace (Vehicle folder)
If Vehicle folder:
So, you are trying to change cars from the original ones to custom. Try this solution:
Find the latest patchday28ng in OpenIV
It should be located in the dlcpacks thru the mods folder. Find the file with vehicles, and put every ytd, and other vehicle files inside. Then head into Gta main directory, into the els folder, and put every xml els file
If the files are addon (DLC) try this:
Find dlcpacks folder, put the folder with cars inside. Head to dlclist and copy one of the lines, and at the bottom paste it, change the name to the file folder located in dlcpacks.
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Let me know if it works!