Get $25,000 In Career Mode:
Pause the game and press L2, Square, R1,Square, R1, L1, Circle, L2, Square, R2, 
Square, R1

Unlock All 18 Trucks and Custom Colors:
Pause the game and at the title screen press L1, L2, R1, R2, Square(2)   
NOTE:this has no effect in career mode.

Unlock All Cheat Trucks :
Enter these codes at the title screen L1, L2,R1,R2,L1,R1
Submitted by KGVandeveld 

Start with $1,000,000 
L2,square,R1,O,R1,L1,O,L2,square,R2,square, R1
Submitted by KGVandeveld