Contact an attorney.

They will probably try to help you get the car back or help you sue the ex for the down payment & any other money you put into the car. The finance company won't "take your name" off of the contract until it is financed through someone else. So if your ex leaves the car under his name, your name will always be on there as the co-s. And also, if the person they are "letting make payments" on the car defaults, it will mess up your ex's credit before it will mess up your credit. They will try to get the money out of him first before they try to get it out of you.

Good luck, but I would at least CONSULT with an attorney & explain the situation to find out what the best course of action to take will be..