California Code, Vehicle Code - VEH § 28080

(a) Every motor vehicle upon which a camper is mounted shall be equipped with an audible or visual signaling device which can be activated from inside the camper and which is constructed so as to allow any person inside the camper to gain the attention of the driver of the motor vehicle. In no event shall a horn, as required by Section 27000, be used to comply with this subdivision.

(b) No person shall drive a motor vehicle upon which is mounted a camper containing any passenger unless the motor vehicle is equipped as required by subdivision (a).