All tracks and cars: 
In the options menu, select secrets and enter:

CFCKGL - unlock B mode and cars 
AIDLAU - unlock all cars 
BSDJUC - unlock all tracks 
FUDWHC - All parts
DMLPOD - All tests
EPDZJF - Mirror tracks

Unlockables 
Win (or come second) the specific championship to unlock the car or cheat 

Mitsubishi Pajero 
2WD (Advanced) 

Lancia 037 
2WD (Advanced) 

Citroen 2CV 
2WD (Normal) 

VW Golf 
2WD (Normal) 

Expert Championship 
4WD (Advanced) 

Ford Transit Rally Van 
4WD (Advanced) 

Ford RS200 
4WD (Advanced) 

Extreme damage 
4WD (Advanced) 

Peugeot 206 
4WD (Normal) 

Audi Quattro 
Group B 

Mirror mode 
Group B 

All tests 
Group B 

Enter the following cheats at the Code Entry screen in the Secrets option section. The following codes need a master or bonus code which will only work on certain discs. You may have to try more than one. 

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 ? 
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 

Unlockable Bonuses:
Come in first or second in the championship listed to unlock the car or cheat. 

Bonus - Championship 

All tests - Group B 
Audi Quattro - Group B 
Citroen 2CV - 2WD (Normal) 
Expert Championship - 4WD (Advanced) 
Extreme damage - 4WD (Advanced) 
Ford RS200 - 4WD (Advanced) 
Ford Transit Rally Van - 4WD (Advanced) 
Lancia 037 - 2WD (Advanced) 
Mirror mode - Group B 
Mitsubishi Pajero - 2WD (Advanced) 
Peugeot 206 - 4WD (Normal) 
VW Golf - 2WD (Normal)