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