Cheats for 18 Wheeler: American Pro Trucker (PS2)

Bonus parking levels
Successfully complete each of the four parking levels to unlock an additional parking level. Successfully complete the bonus parking level to unlock a sixth parking level.

Unlock more parking levels:
Finish the four parking levels to unlock a fifth parking level. If you finsih that as well, you will unlock a sixth parking level as well.

Unlock the Nippon Maru:
At the title screen, hold X and press Start. Dont release X until the mode selection screen appears
-or-
Beat the game with all four characters to unlock the Nippon Maru