What do you do when you need to get your co-signer off your car to trade in but cannot get ahold of them?

I am trading in my car. My ex is my co-signer and has agreed that he would come into town to sign off on the papers so I can trade in my car. That was over 2 days ago! I have found a car and everything is ready to go however I cannot get in touch with him. He is no longer answering my calls. I really need to get him to sign off so I can trade in my car. What else can I do?