Could be better. – Plext: License Plate Messaging Review

This is a good idea but if the person you are trying to contact is not a user no message will be delivered. It would be a nice addition if you could get a plex bumper sticker as a user (possibly with QR code) so people who use the app know the car is connected and the message will go through. Otherwise leaving a note on a car windshield is probably better.
Review by Medic141 on Plext: License Plate Messaging.