What will happen if you move out of the country stop making car payments and register a vehicle in Mexico if the dealer still holds the title?

The person taking the vehicle out of the country would be guilty of a felony according to the state laws where the vehicle was purchased. It could be grand theft auto, stealing by deceit, transporting a stolen vehicle across state lines (which can be a federal charge), and any number of other criminal charges. It is probably possible with enough money changing hands with certain person's to register a stolen vehicle in Mexico.