Enter the following cheats in the Cheat Menu. A sound will confirm correct code entry. After entering the All Tracks code it will automatically exit from the Cheat Menu. The other codes won't make you exit.
Effect - Code
First Additional Greeting at End of Credits - MUNCHKIN
Second Additional Greeting at End of Credits - EDDROOLZ
Third Additional Greeting at End of Credits - EDDIEPOO
Unlock All Levels of Racing - FREEPLAY
Unlock All Tracks - WIBBLE
Unlocks O2 Liveried - UCANDO
Complete season mode with all first place finishes and all objectives completed.