Try putting your car files here. (S:\Steam\steamapps\common\Grand Theft Auto V\mods\update\x64\dlcpacks\patchday21ng\dlc.rpf\x64\levels\gta5\vehicles.rpf\)
(I dunno if this actually works) When you go on the non mods folder usinf openIV, where u put the car/s, a red bar/banner will show. "READ IT" and if it says "this archive folder isnt located at your mods folders" there should be 2 options. 1) Copy to Mods Folder. 2) Show Mods Folder Settings. I pick the1st one. Then after you'll get redirected to your mods folder. Then you can place your cars and start up gta. I think its just the mods folder isnt updated.