You need to talk with a family law lawyer. You need to demand the return of your car in writing. I would call your insurance agent and explain the situation. If he took it without your permission, you may want to consider contacting law enforcement.
First, consider reporting your ex to the police for auto theft, if that is what he did. If you let him take it and he gets into the accident, then you will legally be responsible. If you do not want the car, remove the plates, turn them into the motor vehicle department and notify your insurer to cancel your policy immediately. If you just sit back and do nothing, you are headed for headaches and trouble.