Cal. Code Regs. tit. 13 § 686
Aftermarket lamps, with orientation markings such as "top" shall be mounted in accordance with the markings. Sealed and semisealed optical units shall be installed with the lettering on the lens face right side up. Front and rear reflex reflectors shall be securely mounted on a rigid part of the vehicle with the plane of the lens perpendicular to the roadway and parallel to the rear axle. Side reflex reflectors shall be mounted with the lens face perpendicular to the roadway and parallel to the rear wheels. Aftermarket devices with nonadjustable housings shall be mounted with the base on a horizontal or vertical surface, whichever is appropriate, unless different mounting instructions are included with such devices when offered for sale.
Cal. Code Regs. Tit. 13, § 686
Note: Authority cited: Section 26103, Vehicle Code. Reference: Sections 24012 and 26103, Vehicle Code.