It all depends on what car you are adding. For example, if you are replacing "police" you would need to put the new files in the most current DLC pack that contains the files you want to replace, so for "police" that would patchday4 that was released by Rockstar.
The reason for this is because Rockstar will sometimes release a patch for the game, updating a car or fixing a bug that was associated with it. Thus, causing them to put the new version of the car in a DLC pack.