Your best bet would probably be to file a lawsuit on the debt and if you can obtain a judgment against him, then you may execute against his personal property and seek a garnishment of his wages if he is not exempt from garnishment. Once you have a judgment, you will be entitled to conduct discovery in order to determine what, if any, assets he does have.
If he's still your friend, talk to him first, and see if you can work it out. If not, you will have to sue him and get a judgment to be able to garnish his wages (provided they're not exempt because he's a head of a family), or for the sheriff to go and take the car back (very expensive to do by the way).
Please note that the above is not intended as legal advice, it is for educational purposes only. No attorney-client relationship is created or is or is intended to be created by my answer. You should contact an attorney in your area to discuss your case.