Unless you have a contract with your co-borrower and co-owner that says you have a secured interest in the debt unless and until they pay your back as they promised, and unless you perfected that security interest by filing the requisite UCC forms so that you could foreclose, I think the police are wrong, and that you don't have every right to this "self-help" method of getting the car.
I think you should get a judgment against your co-borrower and co-owner, as my colleague suggested, and then execute on that judgment by attaching the car.
PLEASE READ THIS BEFORE YOU COMMENT, EMAIL ME OR PHONE ME. I'm only licensed in CA. This answer doesn't make me your lawyer, and neither do follow-up comments and/or emails and/or phone calls, and you shouldn't expect me to respond to your further questions if you haven't hired me. We need an actual agreement confirmed in writing before any attorney-client relationship is formed. This answer doesn't constitute legal advice, and shouldn't be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue.