SAVE the game after you win the race, but before it gives you the car(if this is possible).
Then go get the car, and if it's not the car you want, go to the main menu and hiot LOAD GAME, then go back to the place where it gives you the car. Do this until it gives you the car you want.
YOu have to have a PS2 Memory Card. I think you can do this on endurance races, but I'm not positive if you can save after an endurance, only way it to know is to try.