Under the secrets in options you can enter the codes listed below. They are dependent on a 'Secret Access Code', different for each install of the game.

Password - Effect:

FNDDLF - CODE GROUP: 33056 ALL CAR PARTS
TPDSTS - CODE GROUP: 33056 ALL OTHER CARS
RLEVWG - CODE GROUP: 33056 ALL TESTS
QMLRPI - CODE GROUP: 33056 ALL TRACKS
SFKTCP - CODE GROUP: 33056 EXPERT MODE
BEDMSH - CODE GROUP: 33056 MIRROR TRACKS
MBNNOE - CODE GROUP: 33056, GROUP B MODE/CARS
ZCDGEN - CODE GROUP: 34674 ALL CAR PARTS
FEQRJV - CODE GROUP: 34674 ALL OTHER CARS
WBDWYE - CODE GROUP: 34674 ALL TESTS
ZCNGMW - CODE GROUP: 34674 ALL TRACKS
IRZQNT - CODE GROUP: 34674 EXPERT MODE
XOEFIP - CODE GROUP: 34674 GROUP B MODE/CARS
HCHSFD - CODE GROUP: 34674 MIRROR TRACKS
AJOOOU - CODE GROUP: 62297 ALL CAR PARTS
AJOOOU - CODE GROUP: 62297 ALL CAR PARTS
FNDKVA - CODE GROUP: 62297 ALL OTHER CARS
FNDKVA - CODE GROUP: 62297 ALL OTHER CARS
HFDHVM - CODE GROUP: 62297 ALL TESTS
OHZJWH - CODE GROUP: 62297 ALL TRACKS
OHZJWH - CODE GROUP: 62297 ALL TRACKS
SPUFMB - CODE GROUP: 62297 EXPERT MODE
SPUFMB - CODE GROUP: 62297 EXPERT MODE
SMOJYN - CODE GROUP: 62297 GROUP B MODE/CARS
SMOJYN - CODE GROUP: 62297 GROUP B MODE/CARS
ABDLEO - CODE GROUP: 62297 MIRROR TRACKS
UEDIUP - CODE GROUP: 71077 ALL CAR PARTS
ZFBDKH - CODE GROUP: 71077 ALL OTHER CARS
YCDYZH - CODE GROUP: 71077 ALL TESTS
JJRGKX - CODE GROUP: 71077 ALL TRACKS
ACDBEI - CODE GROUP: 71077 EXPERT MODE
APCSTK - CODE GROUP: 71077 GROUP B MODE/CARS
ZMDHKN - CODE GROUP: 71077 MIRROR TRACKS
JUDFUE - CODE GROUP: 76538 ALL CAR PARTS
EDDXVB - CODE GROUP: 76538 ALL OTHER CARS
EUDNKA - CODE GROUP: 76538 ALL TESTS
AHZSIX - CODE GROUP: 76538 ALL TRACKS
CXYIYF - CODE GROUP: 76538 EXPERT MODE
RTNITJ - CODE GROUP: 76538 GROUP B MODE/CARS
VJFWEF - CODE GROUP: 76538 MIRROR TRACKS
DZDKLT - CODE GROUP: 83119 ALL CAR PARTS
LRUTRU - CODE GROUP: 83119 ALL OTHER CARS
ABNEZS - CODE GROUP: 83119 ALL TESTS
DSHRQV - CODE GROUP: 83119 ALL TRACKS
WNDWPP - CODE GROUP: 83119 EXPERT MODE
BUJUYO - CODE GROUP: 83119 GROUP B MODE/CARS
BVHPRU - CODE GROUP: 83119 MIRROR TRACKS
XKDBLU - CODE GROUP: 84419 ALL CAR PARTS
EBDIQQ - CODE GROUP: 84419 ALL OTHER CARS
CIDLCC - CODE GROUP: 84419 ALL TESTS
SXDEBD - CODE GROUP: 84419 ALL TRACKS
EIDYVP - CODE GROUP: 84419 EXPERT MODE
EEHIZA - CODE GROUP: 84419 GROUP B MODE/CARS
YUDWVP - CODE GROUP: 84419 MIRROR TRACKS