In the main menu, pick any of the two race car classes and once you do that, go to Control and start a testing run. Doesn't matter what track you select. In the menu where you actually debrief everything, you'll see a chatbox on the lower left of the menu. In that box, type ISI_BABYFACTORY (All caps) and send it, then exit the race. Once you do, you'll see that you're left with way more money (Around 1-2 billion depending on which class you selected) and most, if not all of the cars, unlocked.

NOTE: You must re-enter this code for each car class if you want to use it for both of them.