If there is equity in the car, sell it now before the dealer repossesses it and then sues you for any deficiency, which could damage your credit further than he already has by failing to pay on the loan. Hopefully, the loan is with the dealer and not with a third party like Wells Fargo...
While I have not seen the agreements you signed and cannot give you legal advice in this forum, I may be able to give you some general information that could be helpful.
It sounds like you signed the agreement with the dealership. If so, you are ultimtely the one on the hook.
Your normal remedy against your ex (assuming you mean boyfriend or girlfriend and not spouse) is a lawsuit for breach of contract.
If that person has no money and the car is in your name, you could consider selling it now to limit the amount of your loss. Talk to the finance department at the dealership.
If the dispute is large enough, contact a local lawyer who might work with you on the fees and help you out.
Although I cannot give you legal advice, I agree with the other attorneys, that you may want to immediately sell the vehicle in order to mitigate any further loss to yourself. However, it sounds like you also have a contract with your ex to pay you. Even if you sell the car, he is still liable to you for any damages based upon that contract. If damages are not too high, I would suggest considering small claims court against your Ex. In small claims, you do not need to hire an attorney and the cost to bring the suit is minimal. Even if your ex does not have the money to pay you right now, a successful judgment against him would be good for ten years and is renewable. If you decide your damages are significant enough to bring suit against the Ex, do not delay in bringing the action because you must file suit within the time frame defined by the applicable statute of limitation. An action for a breach of written contract is 4 years. (California Code of Civil Procedure Section 337).