At the title screen, press Up, Down, L, R, then Select. If you've done the trick correctly, you'll hear an explosion in the background. Now go to the Options menu and choose Mystery Mode. You'll be able to play the whole game from an overhead view.
Battle Cars
Boss Car
To get the Boss Car, press L, R, and Up at the title screen. If you did the trick correctly, you'll hear a rumbling sound. Begin a normal game. When you reach the boss, you'll be driving his car.