To unlock bonus vehicles, you have to enter one of the following codes at the main menu:
Bus: School bus
armytruck: Army truck
semi: Semi tractor
jeepyj: Jeep Wrangler
commanche: Jeep Commanche pickup
vwbug: VW Beetle
vwfb: VW Fastback
vanagon: VW Vanagon
volvo: Volvo station wagon
bmw: BMW
mercedes: Mercedes
miata: Madza Miata
quattro: Audi Quattro
landcruiser: Toyota Landcruiser
snowtruck: Snow plow
drive29: Monolithic Studios Bus
drive30: Limo
drive31: Citroen 2CV
drive36: Cart
drive37: Outhouse
drive38: Tyrannosaurus Rex
drive39: Wagon
drive40: Vendor Stand 1
drive41: Vendor Stand 2
drive42: Vendor Stand 3
drive43: Log
drive44: Wooden crate
drive45: Monorail train
drive46: Hoverpolice
drive47: UFO
drive48: Hovering sewage truck
drive49: Snowy wooden box
drive50: Snowy wooden box 2