I have installed the 2005 cars into the cars.wad file. At first the Sauber and the Jordan cars had the same Sauber livery (that was even before replacing their original liveries).
Then I tried to install a new Jordan livery and what I got was that one into the Ferrari. Now I can't get the Ferrari back!
I don't have any problems with the other cars. Of course I replaced the corresponding gp4 and tex files for each car.
I finally solved the problem. It is the "TeamPatcher" (that comes with all the textures) the one that changes everything, I have just replaced GP4.exe with an old one and everything is back to normal.