To use these codes you must be fast enough to have your time entered on the high score screen (make sure to turn checkpoints on from OPTION screen.)

knacked - Reverse tracks
whoooosh - Nitros (press HORN while racing)
mjcim.rc - Tiny cars
sausage - Bonus cars

Note: After entering the codes make sure you save your game.
Otherwise, you will have to run this each time you play.