California Code, Vehicle Code - VEH § 25270

Any pilot car required by the permit referred to in Section 35780 or 35790, or any vehicle or combination of vehicles subject to the permit if specified in the permit, shall be equipped with flashing amber warning lights to the front, sides or rear.  The pilot car and any vehicles required by the permit to have flashing amber warning lights, shall display the flashing amber warning lights while actually engaged in the movement described in the permit.  The warning lamps shall be removed or covered with opaque material whenever the pilot car is not escorting the movement described in the permit.