During a game, press CTRL ALT SHIFT F7. A box should appear. A list of cheats can be found here. http://www.gamespot.com/pc/driving/midtownmadness/hints.html
This only works for some levels and I did not make it up. You just go to any level that you can get a car with, like level 16. Then you play level 16 or the other levels once dumbly, then you close...