I'm Lucy, and I'd be happy to answer your questions today.
When two people purchase a car together, they have equal rights to that vehicle, unless they sign something saying otherwise. I mention that because your ex is legally entitled to half of the equity in the car, even if you're the only one who has been paying for the past few months (and regardless of who was using or driving the car before that). You have the ability to file a Petition for Partition by Sale, asking the judge to order your ex to do what is necessary to assist you in selling the car to a third party and splitting the proceeds of the sale, if any, after the loan is paid. If you want to trade it in, you can ask the judge to let you buy him out instead. But you will have to pay something in order to do that.
If you'd prefer to avoid going to court, it could help to get the Kelly Blue Book value of the car, look at the amount still owed on the loan, and see if your ex is willing to sign the title over to you if you pay him his fair share.
Also, if he's been driving the car all this time and he's supposed to have been making payments (and you can prove that), you can sue him for the payments you had to make to protect your credit. And in that scenario, it's possible that you'd be able to get him to return the car in exchange for not suing him. If you get a judgment against him, that's only going to make his credit worse.
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