How can I get my C240 Mercedes-Benz back from my Ex-Boyfirend?

Go to his place of employment. It is very possible that he is hiding the car somewhere else. You can repossess the car and as long as you claim it before it is auctioned off, it won't affect your credit. That's how I got my Lexus back from my ex; I gave the finance company his info and they tracked him and caught him. He was held in jail until he revealed the car's location and he didn't want it to get out and possibly affect his job. I paid the fees and got reimbursed in court. The title is in your name; it doesn't matter that he has a key. Teens steal their parents' cars all the time and they have a key. It would be no different than if you were held at gunpoint or carjacked; they would have the key and the car was still stolen. I can tell you most luxury cars do have a GPS tracking device, either through On-Star or the factory through the navigation system. You can also track his cell phone; it has GPS also. Hope this helps.