At the main menu quickly press L1, L2, Triangle, R1, R2 and then click on Grand Prix to get 2,000,000,000 credits. Note that you must have exactly 15,000 credits for this code to work, if you enter the code without having exactly 15,000 it will reset whatever amount you have back to the default 15,000 first, then you must exit back to the main menu and enter the code again to get the full 2,000,000,000.