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I'm sorry to hear this. So long as you don't have a lease arrangement with him, and it sounds to me like you basically just let him use with no specific agreement regarding it, you would be absolutely entitled to return of the vehicle. If you have a set of keys, I see nothing legally wrong (again, assuming he does not have a lease or some legal right to have the vehicle) with you going out there, getting in the vehicle, and driving it away. I would first send him a certified letter explaining that if he doesn't return the vehicle to you by a certain date, that you will take action to recover the vehicle. After the date has passed you might either try self-recovery, or possibly even reporting the vehicle stolen to his local police department, and explaining the situation. They may or may not consider that theft and pursue it criminally (I contend that it becomes theft even though you permitted him to use it, when you send him the certified letter demanding its return and he refuses.) If you are afraid to do either of those things for some reason, you can hire a lawyer in California and file a suit for what is known as "replevin" (return of personal property). A more detailed article on replevin may be found here: http://en.wikipedia.org/wiki/Replevin
Good Luck, and I hope this helps.