Someone has put an airtag on my car

From: AirTag - Apple

"AirTag is designed to discourage unwanted tracking. If someone else’s AirTag finds its way into your stuff, your iPhone will notice it’s traveling with you and send you an alert. After a while, if you still haven’t found it, the AirTag will start playing a sound to let you know it’s there.

Of course, if you happen to be with a friend who has an AirTag, or on a train with a whole bunch of people with AirTag, don’t worry. These alerts are triggered only when an AirTag is separated from its owner."

I don't think it will tell you to whom it belongs

Do you have an iPhone? You can try using it to locate the AirTag.

Once you located it, the only way you can find out who may belong to is if they included their contact information. I doubt seriously, if they done this to track you, that they would provide this kind of information.

I suggest that you take the tag to your local law enforcement agency, and they can work with Apple to try to discover the owner. However, the owner may not be the person who actually placed the tag in your car.

https://www.youtube.com/watch?v=mGh7-IuPRR4