https://articles.latimes.com/1985-09-03/business/fi-24385_1_air-bags The U.S. law started in '86 for manufactures to make cars with 3rd lights. So the real question will be will retrofitting to an older car be required to meet these standards.
Just keep in mind that a 3rd brake light has to be mounted higher than the taillights, and brighter than tail or position lights to be legal.
Just keep in mind that a 3rd brake light has to be mounted higher than the taillights, and brighter than tail or position lights to be legal.