Simply by switching the files in your Atari game files (located in Program Files: Atari: Test Drive Unlimited: Euro: BNK)
You can create your own custom cars, with different interiors, rims, and, in the case of cars such as Ferraris, allow them to be painted any colour you wish.
You must make sure the files you are switching are roughly the same size.
Copy the file containing the Audi S4's exterior
Cut the file of the Shelby GR-1' exterior and paste it somewhere safe
Paste the Audi's interior in place of the Shelby's and rename it exactly as the Shelby file.
Play the game and visit the Ford Showroom. The GR-1 will have the exterior of an Audi! This gives you a 217mph chrome coloured convertible!
Ive experimented many times, and although I've stopped now, I've done many successful switches, including giving the Lambo Miura the body of the "Filler" car, allowing a Ferrari to be custom painted, giving a Mclaren F1 a koenigsegg body, and creating the world's fastest convertible (Wiesmann Roadster with Koenigsegg engine)