Currently I do not know of any controller button combos or usernames to unlock every single car in the game. You will simply have to get them the old fashioned way and that is win and unlock them on your own. I do have a tip for the harder races that specify no special car be driven. When a race states that the cars used is open chose the BMW M5 since it is a slower good handling car that is difficult to spin out and crash with, especially if you are on the very hard mirrored, backward, or night time tracks that you have never raced on before. Another good tip is if you are not in first place and find yourself on an unfamiliar version of a track you thought you already knew is to follow your opponents the first lap to learn where to slow down and turn at.