S.C.A.R.S. 

Cheetah car 
Enter "NRNNRR" as a password.

Cobra car 
Enter "TRTTLL" as a password.

Panther car 
Enter "4MSTTR" as a password. Alternatively, enter "YMSTTR" as a password. 

Scorpion car 
Enter "SDSSRT" as a password.

Unlock Crystal Cup in Grand Prix mode 
Enter "LGSSSX" as a password.

Unlock Diamond Cup in Grand Prix mode 
Enter "CRKKYY" as a password.

Unlock Zenith Cup in Grand Prix mode 
Enter "DZPKKK" as a password.

Master mode and all bonus cars 
Enter "PXPRTS" as a password.

All cars and cups 
Enter "GGWWOO" as a password.

Enable all codes 
Enter "WLLVDD" as a password.
Alternately, press Left, Up, Right, Down, Z, R, Down, Left, Up, Right at the player selection screen.