California Code, Business and Professions Code - BPC § 7029.6

Except for contractors identified in Section 7029.5, every contractor licensed under this chapter shall have displayed, in or on each motor vehicle used in his or her construction business, for which a commercial vehicle registration fee has been paid pursuant to Article 3 (commencing with Section 9400) of Chapter 6 of Division 3 of the Vehicle Code, his or her business name and contractors' license number in a clearly visible location in print type of at least 72-point font or three-quarters of an inch in height and width.