Press UP, DOWN, UP, DOWN, LEFT, RIGHT, LEFT, RIGHT at the "Press Start" screen. "Special Challenge Event" should then appear on the screen. Next go to the main menu, then choose "Challenge Series". Head all the down till you get to number 69, the BURGER KING CHALLENGE. Complete the challenge and you will be able to use all junkman parts on cars you have under " My Cars". Before you apply the special parts for performance, press "A" to get them as high as possible. Doing this gives your car better stats.
At the title screen, enter the code. You should get a confirmation.
Before you sell a car, go to a One-Stop Shop and reset all the exterior body parts (body kit, wheels, spoiler, hood, etc.) to stock. You will get a trade-in credit for the old custom parts and it will not lower the car's value when you sell it in the Safe House.
Watch your tachometer when at the starting line. Get your needle in the blue right when the race starts to get a perfect launch and gain some time against your opponents.