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Dreamcast - 18 Wheeler - American Pro Trucker cheats

Disable timer and unlock all cars, trucks:
Go the main menu, and enter the following button combination: 
Hold L and press A, Y, X, B, Left, Start 

Additional parking levels
Finish all 4 parking levels

Unlock 6th parking level:
Finish the bonus 5 parking level

Unlock Nipon maru:
Finish arcade mode with all 4 characters

Unlock Lizard Tail 
Finish the game with all 5 trucks, including the extra car: Nippon Maru. 

Unlock in trailers in Score Attack, Versus and Arcade mode for all trucks:
Finish the arcade mode with all 4 characters. 

Get bonus parts 
Complete the parking mini-ame, afterwards defeat your opponent in 
any Arcade race.