Enter the Gran Turismo mode. Go into your garage and get into a Dodge Viper R/10 in the blue colour. (you can get this car at Chrisler) go and start the Sunday Cup you will need a B-Licence for this. When you are Racing do the first lap without coming of the Track as soon as you did the first lap turn around and drive the other direction.

Do that one lap without comming of the track once. After you done that pause the game and press L1+R1+cross+up+down and then quit the race. Go to the garage and change your car to anything you like.

If you done this right you should hear a ringing sound under neath your screen it should say "unlimited credits". You will have unlimited credits and all the cars in the gran turismo mode.