Enter the following codes at the bonus screens:
Note: The codes are different from game to game, depending on the bonus number. Try all three and you should find, one will work. 

All Cars: 
XKIMCF, LEZEJD, or SAFREA 
All Championships: 
QTRRDJ 
All Challenges: 
LSDUXH 
All Tracks: 
QTRIKX, TMYTKO, NIQHLT, NPXOOY, MPXOOY, or HZCGYP 
Enhanced Damage: 
DAMAGE 
No Damage: 
IHHBIV, OATRYU, or MMMFJL 
Pro Race Driver Tracks: 
LQJFFA 
Realistic Handling: 
SIM, VIBAKH or LJLZEL 
View Credits: 
CREDITS or XQIXCJ