Press Up, Down, L, R, Select at the title screen. Enable the new ‘Mystery Mode’ selection at the bottom of the options screen.
Drive Boss car:
Press L, R, Up at the title screen. A rumbling sound will confirm correct code entry. Start a normal game. After reaching the Boss, you will be driving his car.
Alternate car colors:
Highlight a car at the selection screen then press A + Select + Start. You will now be able to choose a color for your car.