Okay Axis maN, here it goes...
Example, 5 race series. If you notice after you complete a race, you have the options at the bottom of the screen. Things such as Next, Replay, Exit, Save..., and another Save. The first Save... is to save the replay, don't do that one. Do the Save down at the end before the Exit box. This allows you to save during the series. This is helpful if a power outage happens....
Anyway, I suggest saving after each race anyway. I get sleepy too fast nowadays while racing long races and such. Say you win the first 4 races, that gives you 40 points. The highest points that second could have would be 24. Do the math with one race left, you can start the last race, then quit out at the start line, and you will still win the series. Someone else will get the 10 points, and you will get zero points for quitting a race. Just do the math and you may be able to quit after 7 or 8 races in a 10 race series. Anyway, I just suggest going through all the races except the last one.
Remember, save before hitting Next. After you hit Next, you CANNOT save until after the race. Now start the final race, making sure you are at least 11 points ahead of the second place, press start and quit the race. Then hit Next, you will be awarded the credits for winning the series, then taken to the prize car screen. You can try to use that "Press X" trick if you want. If you get the car you want, good!! Go save, and you're done. If you don't get the car of your choice, press triangle until you get to the main screen, then load game. That will take you back to the series again, ready to begin the final race. Repeat until you have desired car.
This may take a few tries to get the car you want, but just keep trying. Good luck!! I hope this helps, if you need something else, feel free to ask!!! I will explain anything that is not clear for you.