Enter the following cheats at the "Code Entry" screen under the Secrets option. Each game disc has one of a few random "bonus codes"--only cheats that are made for the bonus code that matches your own disc's bonus code will work for you. You can get a different bonus code by turning off your Xbox and restarting the game.

Bonus Code 76538
Group B Mode and Cars: RTNITJ

Bonus Code 34674
All Bonus Cars: ZCNGMW
Expert Mode: IRZQNT
Mirror Tracks: FEQRJV
All Tracks: HCHSFD

Bonus Code Unknown
All Weather: CLRWTR
Group B Mode and Cars: CFCKGL or YFHPGW
All Bonus Cars: AIDLAU
All Tracks: BSDJUC
Expert Mode: HIQNRM
All Parts: FUDWHC
All Tests: DMLPOD
Mirror Tracks: EPDZJF