architect,
I've just downloaded this package and created a new Cars.wad file using WadUpdater 3.4 and I sent you a PM with detailed instructions.
In GP4, go to Options, Graphics, Advanced and select Medium in the Texture box. This should get rid of the white cars.
However, if you, like me, had a Team 11 which now show white, go back to the Cars\cars directory and remove "hi_" from the Team 11 car texture file.
This works for me and the resulting Cars.wad had a filesize of 375MB. My previous one was 679MB.