If it is textures are in TGA, then use GP4masters tex workshop to put them into texfiles and insert them into cars.wad - OR - import them directly into cars. wad with CSC4.
If they are tex files, then import them with CSC4 and insert into the cars.wad - OR - unwad your cars.wad with GP4master, replace the files, and rewad - OR - you can use Wadupdater.
If carshapes do not call for the correct textures after you have put them insto the cars.wad, then see this thread:
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www.grandprix3.com]
It's only after we've lost everything, that we are free to do anything.