In career mode, go to your garage and do something with one of your cars (for example, switch a part or get a different paint job). The game should autosave. Then, go back to your race menu. Go in a sprint and bet all your money on one car that looks the fastest. It it does not win at least three races out of four, turn off the PlayStation 2 and turn it back on. It will load your last autosave. Try the same sprint again until you win then find a different sprint and start the entire process again.