Whenever motor and other vehicles are operated in combination during the time that lights are required, any lamp (except a tail lamp) need not be lighted which, by reason of its location on a vehicle of the combination, is obscured by another vehicle of the combination, but this does not affect the requirement that lighted clearance lamps be displayed on the front of the foremost vehicle required to have clearance lamps, nor that all lights required on the rear of the rearmost vehicle of any combination shall be lighted.