All cars, all tracks
Go to the options and type in "i have the key" note: don't for get the spaces, If it works you will hear a sound.
Reverse tracks
At options window type in "that takes me back" and you should hear a "ding" noise. When selecting a track there shoud be a "forward/reverse" option.
All other cars stop
Go to the main options menu. Type in "remote braking" with the spaces. You should hear a bang. Now if you honk your horn, all of the other cars will put on their brakes.
Race with cop cars
Go to the options screen and type in "I Carry A Badge".
Unlocks all cup races
Go to the options screen and type in "Cup Of Choice".
Nitros
Go to the options screen and type in "lone crusader in a dangerous world".