Review your contract for the car. I bought a (used) car last weekend and signed a clause on the contract that said my purchase decision relied on the dealer providing accurate information regarding the vehicle and its history. If you have a clause like this in your contract, it should be even more of a no brainer than it already is.
To be honest, I'd check that and immediately contact a lawyer. Sometimes a simple call from a lawyer can make even the toughest people change their mind