Enter two player mode, and select one of the unavailable garages. Quit two player mode, and start an one player race without choosing another car to use one of the previously locked cars.
Kingroader Glitch
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.
Mirror Mode
In the Championship mode, you must win all the 3 courses against Deus to get his car. No you can drive the mirror tracks.
Quick Start
When the timer reaches one, hold Accelerate.
Race against Deus
In Championship mode you must win all the 3 courses against Hannya to get Hannya's car. Now you can race against Deus in the Challenge mode.
Race against Hannya
In Championship mode you must win all the 3 courses to get the option to race against Hannya.
Secret passage
If you drive the Downtown track, and you arrive at the top of the hill, drive beneath the tree just before the waterfall. The you drive in a tunnel that is much easier to drive than the normal track and you win allot of time here!
Win each race
You must have the Kingroader ( the yellow Jeep ) and in the configurationscreen: put the aerodynamics to the far right. Your max. speed must be 205 km/ph. When you are going to race, your wheels stick to the ground like there is some sort of glue beneath it. You have ultimate grip on the snowtracks and in turns.
Bonus cars...