Calling the police seems like a really bad idea. I think OP's mother simply misunderstands what being a co-signer means or that their kid is now a grownup and that the car is something their kid paid for and not the parent, different from probably most cases before OP reached adulthood. It's something that can be worked out by talking. Calling the police would definitely forever terminate any familial relationship OP might have with their mother.

-1 I agree with @JoL. Advising the OP to call the police would be counterproductive if the OP values his relationship with his mother and the rest of his family. The person giving this answer admits that he was not on speaking terms with his own mother. Moreover, remember that your mother did you a big favor in cosigning your loan -- although not as big a favor as she seems to think.

@ab2 Please quote where it's advised for OP to call the police, because there is no such thing.

"who actually also holds - in case of a car - the title" is wrong, the title actually stays in the possession of the lender until the loan is paid off.

@Agent_L ".....when my mother was still alive, I would have called the police" in the answer above.

@ab2, that's a different statement from "you should call the police". Obviously, these are different people, with different parents and different relationships therewith.

@CharlesDuffy but in the situation of someone asking another for advice, where the person providing the answer says "I would do X" or "I would have done X", it is much like saying "You should do X". As Agent_L said, adding something like "You should contact her first" would be a welcome addition to the answer.

@fyrepenguin: TomTom immediately qualifies it with "But then, we were not on talking terms". Your reading is strained at best, ridiculous at worst.

@BenVoigt some US states (such as Maryland) the lendee still holds the title and owns the car, but there is a lien on it from the lender that prohibits any transfer of ownership until the lien is removed (debt is paid). The lien also provides a pathway for seizure of the vehicle if the debt is not repaid according to its terms, even with the “owner” holding the physical title.

@Bakna: It looks like Florida is a state where the owner does not receive the title until the loan is paid off

@BenVoigt one of the many reasons why I will never live there. According to my friend in FL a document proving who the lien/loan on the vehicle was granted to is another way to prove “ownership” to the authorities.

If we're going with 'this is what TomTom would have done in this situation' then this pretty much a non-answer.

@jcm If this were IPS I might agree, but getting your stolen things back is economically sound advice.