Enter the following cheats at the "Code Entry" screen under the Secrets option.
Note: 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.

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

Bonus Code 84419

• Group B Mode and Cars: EEHIZA
• All Bonus Cars: EBDIQQ
• All Tracks: SXDEBD
• Expert Mode: EIDYVP
• All Car Parts: XKDBLU
• All Tests: CIDLCC
• All Mirror Tracks: YUDWVP

• Test, Mirror Tracks: Win the Group B Rally Championship.
• Audi Quattro Group B car: Win the Group B Rally Championship.
• Ford Escort RS1600 car: Reach the fifth event in the 4WD Championships, on Normal difficulty.
• Ford Transit car: Get 1st or 2nd place in the 4WD Championship, on Advanced difficulty.
• Lancia 037 Group B car: Win the 2WD Championship, on Advanced difficulty.
• Mitsubishi Pajero car: Get 1st or 2nd place in the 2WD Championship, on Advanced difficulty.
• Peugeot 205 Group B car: Win the 4WD Championship, on Normal difficulty.
• Peugeot 206 4WD car: Get 1st or 2nd place in the 4WD Championship, on Normal difficulty.
• Subaru Impreza 228 STI car: Reach the fifth event in the 2WD Championship, on Advanced difficulty.
• Volkswagen Golf 2WD car: Win the 2WD Championship, on Normal difficulty.