All Reward Cars:
     At the Options menu, hold L1 + R1, then press X, Circle, X, Circle.

Fast Cars:
     At  the  Options  menu,  hold L1 + R1, then press Square, Square,
Square, Square.

Invincible Car:
     At  the  Options  menu,  hold  L1  +  R1, then press Triangle, X,
Triangle, X.

High Acceleration:
     At the Options menu, hold L1 + R1, then press Triangle, Triangle,
Triangle, Triangle.

Jump Car with Horn:
     At  the  Options  menu,  hold L1 + R1, then press Square, Square,
Square, Triangle.

One Hit Kills:
     At the Options menu, hold L1 + R1, then press Triangle, Triangle,
Square, Square.

All Cameras:
     At  the  Options  menu,  hold L1 + R1, then press Circle, Circle,
Circle, X.

Show Credits:
     At  the Options menu, hold L1 + R1, then press X, Square, Square,
Triangle.

Show Speedometer:
     At the Options menu, hold L1 + R1, then press Triangle, Triangle,
Circle, Square.

Turn Secret Car into Red Brick Car:
     At  the  Options  menu,  hold L1 + R1, then press Circle, Circle,
Triangle, Square.

Grid Mode:
     At  the Options menu, hold L1 + R1, then press Circle, X, Circle,
Triangle.

Trippy Mode:
     At  the  Options menu, hold L1 + R1, then press Triangle, Circle,
Triangle, Circle.

Bonus Cars:
Level 1: Rocket car by gold house
Level 2: Monorail car on monorail track by station
Level 3: Boat car on the big ship in the harbour (c spanker) 
         in the green box on the pile of green boxes
Level 4: Quad bike in caravan park
Level 5: Monster truck on ledge by the front of stadium
Level 7: RC car on top of krusty burger by the big accident