2010 Pennsylvania Code
Title 75 - VEHICLES
Chapter 43 - Lighting Equipment
4304 - Obstructed lights not required.

     § 4304.  Obstructed lights not required.
        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.