If the death of, or bodily injury to, a person is caused by, or arises out of the use of, an insured motor vehicle while a trader's plate is attached to the vehicle, the third-party policy for motor vehicles to which the plate is attached is (to the exclusion of any other third-party policy for the particular vehicle) the policy under which anyone whom the policy purports to insure is insured in relation to the death or bodily injury.