The Key to the Cars:

As I noted elsewhere, the key to the career mode is the bus station. The key to the top cars, on the other hand, is the Challenge Series, particularly the Burger King Challenge. If you've managed to complete the career mode, you'll have no trouble with the challenge series, which opens up free cars with ultimate performance. The key to free junkman parts is the Burger King challenge, which is surprisingly easy. The series, including the BK challenge, is 70 races, driving everything from a taxi cab to the cement mixer. This might actually be a quicker way to success online.

After you've completed the challenge series, restart the game. At the CLICK to continue screen, go to your keyboard and enter, "burgerking." You'll  see a notice that the BK challenge has been added. When you've completed the Challenge, your "my cars" section will now contain free junkman parts. Choose any car, check "optimize performance" and then add the junkman parts. You're ready to go.