Using GP3master, extract your cars.wad in some temporary folder, name it "whatever"
From Renault.zip unzip all *.gp4 and *.tex files into your whatever/CARS folder.
When it asks to overwrite, say yes. If it doesn't ask to overwrite, then you're unziping it who knows where.
When you've overwrited old files, recompile back into cars.wad
Overwrite your old cars.wad with this new one.
It's obviously not the problem in Renault file, because it was tested and installed many times. You'd better describe how are you installing it, then I'll tell what you're doing wrong.
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Bojan Tarticchio