California Code, Vehicle Code - VEH § 24605

(a) A tow truck or an automobile dismantler's tow vehicle used to tow a vehicle shall be equipped with and carry a taillamp, a stoplamp, and turn signal lamps for use on the rear of a towed vehicle.

(b) Whenever a tow truck or an automobile dismantler's tow vehicle is towing a vehicle and a stoplamp and turn signal lamps cannot be lighted and displayed on the rear of the towed vehicle, the operator of the tow truck or the automobile dismantler's tow vehicle shall display to the rear a stoplamp and turn signal lamps mounted on the towed vehicle, except as provided in subdivision (c).  During darkness, if a taillamp on the towed vehicle cannot be lighted, the operator of the tow truck or the automobile dismantler's tow vehicle shall display to the rear a taillamp mounted on the towed vehicle.  No other lighting equipment need be displayed on the towed vehicle.

(c) Whenever any motor vehicle is towing another motor vehicle, stoplamps and turn signal lamps are not required on the towed motor vehicle, but only if a stoplamp and a turn signal lamp on each side of the rear of the towing vehicle is plainly visible to the rear of the towed vehicle.  This subdivision does not apply to driveaway-towaway operations.