18 Wheeler: American Pro Trucker Cheat Codes for Gamecube

Hint-get more points
On Arcade mode if you see a bonus car keep hitting it to get more points.
submitted by: cheatmaster

More time:
Go into the options menu, select "Difficulty" and set it to "Very Easy". Next, save the game in the options menu and return to the main menu. Select arcade mode and you will have more time in each level, until you reach into the last level and complete the game.
Submitted by: Sachiel

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.

Nippon Maru:
Successfully complete arcade mode with all four characters.
Submitted by: Sachiel

Bonus parts
Go to arcade mode and select any truck. Get to your destination ahead of your rival , then complete the bonus parking mini-game to win bonus parts. Note: This works on any destination.

Shifting
For Highway Cat and Stream Line, shift up when at about 50 mph.
Submitted by: Sachiel

Bonus trailers:
Successfully complete arcade mode with all four characters to unlock two bonus trailers in score attack mode, versus mode, and arcade mode