Go to top right corner of the Oakwood in Extreme mode. You will see two phone booths. Go to the one near the curve to the left, and not the one opposite the Oakwood High School. You will see a house to the left of it with a dark blue door. Get near it and press . Get back to your house in Oakwood and save the game. Quit the current game and start Free Ride mode. You can then select any car. Note: Nothing will say that you have unlocked all Free Ride Xtreme cars, but if you check the Free Ride car selection screen, the cars should be listed there under "Prototypes".