California Code, Vehicle Code - VEH § 24616

(a) A motor vehicle may be equipped with one or two rear-facing auxiliary lamps. For the purposes of this section, a rear-facing auxiliary lamp is a lamp that is mounted on the vehicle facing rearward. That lamp shall meet the photometric and performance requirements of the Society of Automotive Engineers Standard J1424 for cargo lamps.

(b) A rear-facing auxiliary lamp may project only a white light, with the main cone of light projecting both rearward and downward. The main cone of light shall illuminate the road surface or ground immediately rearward of a line parallel to the rear of the vehicle for a distance not greater than 50 feet. The main cone of light may not project to the front or sides of the vehicle.

(c) A rear-facing auxiliary lamp may be activated only when the vehicle is stopped. A vehicle equipped with a rear-facing auxiliary lamp shall also be equipped with a system that allows activation of the lamp only when the vehicle is in the “park” setting, if the vehicle is equipped with an automatic transmission, or in the “neutral” setting with the parking brake engaged, if the vehicle is equipped with a manual transmission.

(d) A vehicle equipped with a rear-facing auxiliary lamp may have an activation switch accessible to the operator from the rear of the vehicle.