What part of the license plate - the vanity number of the "National Defense Medal"? Most states have license plates for campaign service medal veterans - and some even are reduced or no cost vehicle registrations. Your DD-214 is magic. If you are still active, then simply the award letter endorsement on command letterhead suffices.
Another reason to ensure your DD-214 reads just the way you want it to to reflect how you want your years of service to be recognized.