Go to the madgistrate court and explain it to the judge. Have the judge/ judges office place a call to the individual. IF he doesnt answer have the court place a writ of possesion on the car. The cops will deliver the writ to him personally for an appear to court. Keep a log of all phone calls and transaction paperwork.

DO NOT BE A HARD ASS and try to steal your car back. It will only result in you being arrested. Let the court system do there job. It will take a little longer but will result in the best results. I had a gent place a deposit on a truck i sold. He paid 1k down and was going to make the rest of the payments in $200 a month increments. He never paid. I took him to court. Judge ruled in my favor and I got to keep the 1k deposit. Judge told him to grow up and become a man.