I have serial number or unwanted airtag found in my car. How do I find owner?

If you have an iPhone. Hold the AirTag near the it and it should provide you with some information about it. If the owner, enabled Lost Mode, they may have included their contact information. If not, then there is no way to contact them. Having the serial number does not help.

Ref: What to do if you find an AirTag - Apple Support

Did you ever find the owner I went to the police and Apple no help for either one of them

Yes