Well you've done something wrong then!
Firstly, have you got the texture qulaity on high? Most cars now normally need that.
If you have, just use wad updater, no need for the rest. Instert the car shape files, and then put the textures in
one by one. If you try and put them in together, I've found it frequently doesn't do it properly. Then delete the files again (you have to delete them everytime you've updated to wad - normally I update, then go into game and if it's messed up, delete them and then restart). Make sure you've read the readme. It might want you to change the texture names.
It should work if you do all that.