Access the game's Main menu screen. Imagine the four menu options
are numbered from top to bottom 1-4, as follows:

Championship: 1 
Versus: 2 
Setup: 3 
Credits: 4 

     To  enter  the  code, highlight the option with the Control Stick
and  press the Z Button to select. Highlight the options in the proper
numerical  sequence  to  unlock the code. If you have entered the code
correctly, you will hear a revving sound and a smiley face will appear
at the bottom of the screen.

Normal Car Sequence:
4, 1, 1, 2
     Allows players to access all the normal cars.

All Bonus Cars Sequence:
4, 4, 2, 4, 3, 1, 1, 1, 2 
     Allows  players  to  access  the  bonus  cars, along with all the
normal cars.

Season Four Sequence:
2, 1, 1, 4, 3, 3, 1 
     Allows players to start in season four.

Season Five Sequence:
3, 1, 4, 2, 2, 3, 1, 2, 4, 1 
     Allows players to start in season five.

Season Six Sequence:
4, 3, 2, 1, 1, 4, 1, 2, 3, 1, 4, 3, 3 
     Allows players to start in season six.

Alternative Credits Sequence:
3, 3, 1, 2 
     Allows players to see the credits normally seen at the end of the
sixth season.

Wienermobile Sequence:
4, 2, 3, 1, 2, 2 

NP Car Sequence:
1, 4, 2, 3, 2, 1, 3, 4 

3-D "N" Logo Car Sequence:
4, 2, 2, 1, 4, 1, 2, 1, 4, 3, 2, 3 

Open Dump Truck:
2, 1, 0, 2 

Activate the donkey in the cage:
4, 1, 1, 2, 3 

Quick Cash:
     First  start  a  new game and win the first race, when the screen
that shows how much money you have comes up press a on one of the cars
you  have  (Zithout  selecting  that car to play the next level.) Then
press  a  on  another car you have.Do this until you get the amount of
money  you  want. Note: You must have at least two vehicles to do this
code

Bonus Track:
     Finish the 6th season in 4th or better using championship mode to
get the Space Truckin track.