How do you unlock the cars in dead rising 2

The Show cars require keys that you buy from the Pawn Shops. Pawn keys DO carry over between gameplays. Other vehicles must be re-earned every time you restart.

SHOW VEHICLES
Vehicle (location) - Pawn shop location - Cost
Chopper (Yucatan Casino) - Palisades Mall - $1,000,000
Hummer (Shamrock Casino/Silver Strip) - Silver Strip (straight across) - $2,000,000
Roadster (Royal Flush Plaza) - Royal Flush Plaza - $500,000

OTHER VEHICLES
Vehicle - location - requirement
Dirt Bike - Silver Strip - Mission: Meet the Contestants
Ramster Ball (in story, not TIR) - Fortune Park - Environmental minigame: Motion Madness (level 3)
Security/Maintenance Cart - Underground - Receive Underground Access Key

Note: the Bike is the only vehicle that can be customized. To customize it you must use Leon's Trailer at the far East end of Silver Strip.