Go to main menu (where you can select free drive, trophy etc.) and press SHIFT. It will bring up a box to enter cheats. Enter any of the codes listed below:

ALLCARS      - All vehicles
ALLTROPHIES  - All Trophies Access 
ALLTIRES     - All Tires
ALLRDIFFERS  - All Rear Differentials 
ALLFDIFFERS  - All Front Differentials 
ALLENGINES   - All Engines
RABACAR      - Raba Truck 
BIGFOOT      - Toyota Hilux Bigfoot 
ALLCARS      - All Cars    

Thanks to Revolution readers Hurricane and KuLa!