There is a continuous side quest that you can do in Wellspring that is given by Sandy the bartender. She asks you to destroy bandit cars when you are out in the wasteland. Every bandit car you destroy will give you a skull.
If you use the max items option in the trainer it will multiply the skulls to 99. When you go back to visit Sandy she will take the skulls and give you 99 racing certificates.
That is how I was able get all the upgrade for the different cars.
[Edited by fullmetal5550, 10/12/2011 8:10:37 PM] The races are kinda tough but the infinite boost option does make them much easier.
[Edited by fullmetal5550, 10/12/2011 8:25:36 PM]