Of course you donīt deleteall. My english isnīt very well so i canīt make a very good guide, but iīll try:
there you find the folders of every car (some are from "most wanted" or "carbon", you cannot use them)
then you choose the folder of the car youīre driving in world and make a safety copy (on destkop)
And now you have to delete the two datas in the originaly folder
Right now there are some limits, you canīt fid in the data of every car in your carīs folder....i think the car has to be under your level or buyable at car dealer...
For example you have the Corvette z06 ingame, so you are lv 35 or higher, so you can put the gallardo datas in your corvette folder. Then youīll see that you have 4wd power and can drive over nailbands.
Last step (except example): You copy textures and geometry of the car you wanna have in yours and put them in your original car folder where you deleted those.
I hope you can understand
And you might think itīs just for visible things, but for me it works.
My car also doesnīt get invisible or anything else...Should work by you, too.