mistake of fact

You may be able to get a court to rescind the contract, i.e. let you return the car for what you paid (plus interest), minus the amount of wear and tear on the car since you bought it, based on the dealer's fraud.  Fraud is a knowing misrepresentation of fact which is reasonably relied on by the other party and causes it damages.  Here, I don't think you'd  have a problem convincing a judge or jury that a car dealership should have know what model it was selling.  The problem could be "reasonable reliance."  If the model was shown on the car's body, a judge or jury may think that your wife should have known what she was buying, and wasn't reasonable in relying on the contrary statement of the dealership.  Overall, however, I'd say you have a pretty good case based on the facts as you've represented them.