When the game is loaded, press and keep holding X past the first screen and into the mode screen. You'll be able to choose the Nippon Maru truck.
Beat arcade mode with all of the drivers. You'll get 2 extra trailers to play with in the other modes.
There are 6 parking levels in all. If you complete the first 4, the next one will be unveiled, which will unlock the last one once it's completed.
To unlock the first bonus level, complete all four parking levels.
To unlock the second bonus level you must complete bonus level one.
To unlock the Lizard Tail, you must complete the game with all five trucks.
To unlock trucker Nippon Maru, you must finish Arcade Mode on any difficulty setting with all four original truckers.
To unlock the two new trailers, you must finish Arcade Mode with all four original truckers on any difficulty setting to earn two new trailers to use in Score Attack Mode.