Half a year? That's a long time. I would definitely go through the daughter to get to the owners. If they have the title I think they can just sign the back to transfer ownership to you.

Are the owners of the vehicle still on the lease/rental agreement? If they are no longer tenants in a legal sense (ie. they have a right to occupy the residence) then anything they left there after whatever period NY law prescribes before property is considered abandoned, is yours to dispose of. Also, code enforcement might have something to say about it as a nuisance.

On the hand, if they pay regularly, and the car is out of the way and not a hazard, just keep collecting rent and live with it.

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