Drive the Porsche:
On the Car Select Menu, choose car "F", press and hold Left + Right + Y, and press Right. The "G" car should appear.
Drive the Truck:
On the Car Select Menu, choose car "G", press and hold Left + Right + Y, and press right. The truck should appear.
Hidden Obstacle Course:
On the Mode Select Screen, highlight each mode and press B. After you do the last mode, the screen will cycle through each mode and flash. Highlight the Time Trial Mode, and press X. Press and hold the X button, and press A. Select your car and the first course. You should then be on an obstacle course.
Thanks to Revolution reader Jason Beeman!!