If you use easy wad, open cars.wad using the gui option (right click on cars.wad and pick the top option) then copy the tex file for driver number (must be named correctly ) ie driver1_1.tex and driver1-2.tex files into the cars folder and make sure you have the correct helmet shape for the files this is called car_helmet_00.gp4. if you are not sure about the helmet shape then use GP4biulder2015 and open the helmet and convert the driver files using simtex (don't forget to flip them first in the conversion) so you can see how the driver helmet looks before you put them in the cars.wad fie. note
you can only have one helmet shape for all the drivers helmets so might have to pick the one that looks the best, good luck
regards to all