The cheat above didn't worked with my japanese version of ''Wild Wild Racing'' so I experimented a little bit with the buttons!
At the Main-Menu select ''Options''. Now at the Options screen press and hold SQUARE, then press UP, TRIANGLE, DOWN, TRIANGLE, LEFT, RIGHT, LEFT, RIGHT, TRIANGLE. If you entered the code correctly you will see the bar ''SECRET'' below ''Reset Options'' in the Options-Menu! Now return to the Main-Menu, select Single Player, and then enter ''NORTHEND'' as a name. If you entered the code correctly, a voice will say, ''Wild Wild Racing!'' Return to the Main-Menu, select Options, then select ''SECRET''. At the Secrets screen, select ''TOP SECRET''. There you can turn on to have all cars, all upgrades, etc....