CA Veh Code Section 38345

When operated from one-half hour after sunset to one-half hour before sunrise, each motor vehicle which is not in combination with any other vehicle shall be equipped with at least one lighted red taillamp which shall be clearly visible from the rear.

(a)

Every such vehicle or vehicles at the end of a combination of vehicles shall be equipped with one lighted red taillamp when operated from one-half hour after sunset to one-half hour before sunrise.