Cheat Mode
     At  the  Press Start screen, press L + R, C-Left, C-Right, L + R.
Press Start and at the Mode Selection screen, hold Z and press L until
the phrase "Cheat Mode" appears at the bottom of the screen.

Bonus Cars
     At  the  Press Start screen, press L + R, C-Left, C-Right, L + R.
Go to the Options, hold Z and press L.