Enter 2 player mode, and select one of the unavailable garages. Quit 2 player mode, and start a 1 player race without
choosing another car to use one of the previously locked cars.
If you want a vastly improved car, choose the Yellow Kingroader Jeep and adjust the aerodynamics so that the maximum speed is 205kph. For some unexplained reason, the Kingroader will become faster than virtually every other car in the game, with better acceleration and superb handling. You'll hardly ever slide on the road and rarely be overtaken.
When the timer reaches 1, hold Accelerate
To use a bonus car that is locked in the garage, highlight its garage door and press B. SELECTED will appear over the door and you will have selected the hidden racer, you cannot edit the vehicle's settings via this method.