On the race with the castle and dirt road to the side, there will be two rock formations that your car drives under. Immediately after driving under the second formation, make a hard left turn. If done correctly, your car will pass through the wall and reach the short cut.
Complete championship mode under the "Novice" and "Expert" difficulty levels.
At the Main screen, press R(6x).
Complete one of the following series with the corresponding setting to enable a bonus car.
Arcade Basic series (Novice)
Ferrari 512 TR
Arcade Pro series (Novice)
Bugatti EB110 GT
Normal Championship series (Novice)
Arcade Basic series (Expert)
Dodge Viper GTS
Arcade Pro series (Expert)
Normal Championship series (Expert)