That's for the .rpf files inside your GTA 5 directory.
When you replace the vehicles, click the "file" next to "edit" at the top. Press rebuild in that menu. Once that is done, it should show a line in the rebuild tool stating the name, location, and type of modified content. Double click that to make sure it is highlighted, then press rebuild. Once that is complete, it will say "open rebuilt archive?", press yes. Then go back to "file" at the top and select close all archives. Then close open iv and make sure it is not opened when you are in-game. This has happened to me before, so I used this method and it started working. Hope this helps.