Before you can do anything, you must win the Burger King Challenge. When you win 
it, you will unlock all unique parts (also called Junkman parts). Get any car 
and put all the parts except performance. Once you have your car the way you 
desire, go to the performance shop. Max out your car by pressing X. Then, put 
the Junkman parts on. Max out your car again. Then put on the Junkman parts 
again. Your car should be fully upgraded.