First, pull out your divorce decree and make sure which vehicle was given to you in the divorce. Then, if the car is yours, and if the car is where you can get to it without breaking into anything, and if you have a key, then go pick it up. If any of the above is not true, you'll have to sue your ex to get the car back.
I recommend that in the future, you not depend on your ex to make payments on a car that you are depending on for transportation.
I agree with Ms. Walter's response to your post. I strongly suggest that you do what she advices. If the car you received via the divorce decree is not the car in question, then consult with your family law attorney or get one.
My response herein is an attempt to give you general information and direction and is not intended to constitute an attorney-client relationship as perceived by state law.
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