I have a airtag in my car my ex put how do I find it

Tap the "Instructions to Disable AirTag" button:

To disable the AirTag and stop sharing your location, tap Instructions to Disable AirTag and follow the onscreen steps. If you feel your safety is at risk, contact your local law enforcement who can work with Apple. You might need to provide the AirTag or its serial number. 

What to do if you find an AirTag or get an alert that an AirTag is with you - Apple Support

Check the information in this support document, What to do if you find an AirTag or get an alert that an AirTag is with you - Apple Support especially the part about AirTag found moving with you.

If you cannot play a sound, the Air Tag may no longer be with you. And, it may also be a false positive hit from a friend or someone else in the neighborhood (might not be likely). I'd wait and see if it alerts again. If you believe someone did place an AirTag... there are only so many places one could access without full access to the vehicle. Gas tank cover panel, or taped to the frame. Most anywhere else would fall out. You can certainly call the police too if you fear for your safety, but there is likely little they can/will do except to possibly help you rifle through your car.

You would see this:

What to do if you find an AirTag or get an alert that an AirTag is with you - Apple Support

You would be notified if an AirTag not registered to you is traveling with you.

Did you tap on the tab that says Instructions to Disable AirTag??