akira: (heh, I have all the books BTW, at least the colorized Epic version).
For championship series where there is a multiple-choice prize car, you must race the Series Race to win the prize car. If you run the single races you will get prize money each time, but never the car. You may re-run the Series Race as many times as you wish, and you will receive one of those cars each time. The car is randomly awarded so you may get more than one of the same car, or never get one of the other cars. You can use the save-and-reload trick until you do get the car you want.
For the Cup races where there is no series, and only one prize car, you will only recieve one copy of that car no matter how many times your run the cup races. You will get prize money every time.
For Enduro races, you may re-run as many times as you wish and you will get one of the multiple choice cars every time. However there is no trick available to make sure it is the one you want.
Hope this helps.