Once you have lost your car, Mia will take you to a garage. Of the cars you can buy, get the Chevy Cobalt SS. (Important!) Do NOT modify it! Just race it as you would normally. Do everything you need to do to race Sonny, and when you race him, you should win both races. It might take a couple of tries, but when you do, and when you choose the markers, choose the Left and Middle mystery ones. One will probably be the pink slip to his car. When you get his car, sell the Chevy, and max the Golf's performance. Easy!