In the main menu, press Shift and then type in:

Unlock All Cars = ALLCARS

Unlock All Engines = ALLENGINES

Unlock All Front Differs = ALLFDIFFERS

Unlock All Rear Differs = ALLTRDIFFERS

Unlock All Tires = ALLTIRES

Unlock All Trophies = ALLTROPHIES

Unlock Big Foot = BIGFOOT

Unlock Raba's Truck = RABACAR