(27600). No person shall operate any motor vehicle having three or more wheels unless equipped with fenders, flaps, or other devices, at least as wide as the tire thread, to effectively minimize the spray or splash of mud or water to the rear. This section does not apply to vehicles exempt from registration, or registered prior to January 1, 1971, and which have an unladen weight of under 1,500pounds. (Does not apply to motorized bicycles.)
I've been pulled for this more times than I can count. Going stricktly by the VC you must have Fenders or flaps, or other devices. As long as you can prove beyond a reasonable doubt that your tires are covered by your fenders, mud flaps are not required. Tell the cop to eat a bag of dicks.