No offense, it sounds like your ex is a real creep. You have a valid Court Order. Why don't you file a Cross Complaint against him for the arrearage? If he is working, you might want to even think about going to Family Law Court and get the child support increased and also you should attach his wages. If you want to be very aggressive, you could sell the car, apply the money towards the amount he owes you and show the court the receipts and the child support Order to justify your actions.
Unless you catch the judge on a very bad day, I cannot imagine that he is going to allow your ex to obtain a car, not pay past and current due child support. If I was the judge, which I have served many times in Small Claims Court as pro tem judge, I would say to the guy, " You can't be serious".
I hope this is helpful.
John N. Kitta