Upon inputting any of the codes below, there will be some sort of audio 'signal' to let you know that the code worked.

Showroom:

At Title Screen, hold L2+R1 & press Triangle, Triangle, Up, Up, Left, Right, Down, Down, X, X

Big Cars:

At Menu Cube in Main Screen, hold L2+R2 & press Up, Up, Up, Square, Square

Small Car:

At Menu Cube in Main Screen, hold L1+R1 & press Down, Down, Down, Circle, Circle

Boss Code:

At Menu Cube in Main Screen, hold L2 & press Left, Right, Down, Up, Square, Circle, X, Triangle

Total Chaos Screen:

At 1P Title Screen, hold L2+R2 & press Down, Right, Down, Right, Circle, Circle, Circle

Invincibility:

Pause the game, hold R1+R2 & press Up, Left, Right, Down, Triangle, Square, Circle, X

Infinite Weapons:

Pause the game, hold R1+R2 & press Left, Up, Right, Down, Square, Triangle, Circle, X

Unlimited Turbo:

Pause the game, hold R1+R2 & press Down, Down, Down, Circle, Circle, Circle

Unlimited Cash:

At Menu Cube in Main Screen, hold L2+R2 & press Left, Left, Right, Right, Square, Square, Circle, Circle

Change Car Color:

At Menu Cube in Main Screen.
Alt.Color1: Hold R1 & press Square
Alt.Color2: Hold R1 & press Triangle
Alt.Color3: Hold R1 & press Circle