Select 'Options' at the Main screen and then press X on 'Game Status', pick the player you want to use with the X button and then press the L1 + R1 buttons together. Then enter any of the following codes and if you have entered it correctly you will hear a voice say 'Funky'. When you have entered a code you must press L1 + R1 again to be able to enter another one.

Unlock ALL Cars:
LLA_KCOLNU

Unlock Floating Cars:
NO_GRAVITY

Unlock Heavy Cars:
GO_RAMMING

Same Cars in Multiplayer:
MIXED_CARS

Unlock Invulnerability:
ELBICNIVNI

Unlock Hard Mode:
UNDER_FIRE

Unlock Slow Motion Mode:
GO_SLOW_MO

ALL Cars will be Fully Upgraded:
LLA_DORTOH

Unlock ALL Endings:
LONG_MOVIE

CPU Picks Features:
QUICK_PLAY

Unlock Monster Wheels:
GO_Monster

No Wheel Attachment Icons:
DRIVE_ONLY

Quick Vehicles:
MORE_SPEED

Unlock Super Missles:
BLAST_FIRE

Unlock Rapid Fire:
RAPID_FIRE

Suspended Cars:
JACK_IT_UP

No Enemies:
HOME_ALONE

Select the 'Options' Screen at the Main Menu and then press the 'X' button on 'Game Status'. Choose your player with X button and then press the L1 + R1 buttons simulaneously. Then enter the code the following codes to unlock the corresponding effect. If the code has been entered correctly you will hear a voice say 'Funky'. Each time you complete a code you have to press L1 + R1 again to enter a new one. Unlock ALL Cars:
LLA_KCOLNU

Unlock ALL Cars Fully Upgraded:
LLA_DORTOH

Unlock CPU Picks Features:
QUICK_PLAY

Unlock Invulnerability:
ELBICNIVNI

Unlock Floating Cars:
NO_GRAVITY

Unlock Hard Mode:
UNDER_FIRE

Unlock Rapid Fire:
RAPID_FIRE

Unlock Heavy Cars:
GO_RAMMING

Unlock Monster Wheels:
GO_Monster

Unlock Super Missles:
BLAST_FIRE

Unlock Quick Vehicles:
MORE_SPEED

Unlock Same Cars in Multiplayer:
MIXED_CARS

Unlock Slow Motion Mode:
GO_SLOW_MO

Unlock Suspended Cars:
JACK_IT_UP

Unlock ALL Endings:
LONG_MOVIE

No Wheel Attachment Icons:
DRIVE_ONLY

No Enemies:
HOME_ALONE