California Code, Business and Professions Code - BPC § 19236

(a) Each household mover shall display on each vehicle operated by it an identification symbol in the form and in accordance with rules and regulations prescribed by the bureau. The identifying symbols displayed by household movers subject to the Interstate Commerce Commission Order Ex Parte No. MC-41, Identification of Motor Carrier Vehicles, November 17, 1954, effective January 3, 1955, shall serve in lieu of the display requirements of this section, if the identifying symbols have been recorded by the household mover with the bureau.

(b) For motor vehicles first registered in this state on or after January 1, 1985, the identifying symbol shall be displayed on both the left and right doors of the cab of the vehicle.