California Code, Business and Professions Code - BPC § 7029.5

Every C-36 plumbing contractor, C-45 sign contractor, and C-57 well-drilling contractor licensed under this chapter shall have displayed on each side of each motor vehicle used in his or her 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 name, permanent business address, and contractor's license number, all in letters and numerals not less than 1 1/2 inches high.

The identification requirements of this section shall also apply to any drill rig used for the drilling of water wells.

Failure to comply with this section constitutes a cause for disciplinary action.