You'll need an Xbox Live account for this exploit. First you go to the Game Marketplace, choose to find PGR3 Add Ons and there should be a free Aston Martin V8 Vantage for download. Download it and, when it's finished downloading, leave the marketplace and start the game. Now Go to Solo Career and enter your current Garage. Choose "Buy cars" and buy The Aston Martin V8 Vantage. Since the car is free, you can repeatedly buy it and fill every garage until you get to Tokyo Two, the last garage possible.