To unlock a car you must have started a race with it in championship mode. But instead of going through all 22 cars, save the game when the season ends and the next time you load a new season starts. Pick a car to go into race 1 then quit. The car is now unlocked. Keep doing this for every car and then of every season.
Mirrored Tracks
Successfully complete championship after unlocking all 22 cars to unlock mirrored tracks.