If there is not power past the splice at the first running light, inside the frame, the wire may be disconnected, so power is not feeding on through to the more rearward lights. I would try pulling the running light that is working and checking the connection that is there. If this is not the problem I would then run a new piece of wire, item # 12-1-1, through the trailer frame from the first light to the second and splice it in where the original wire was spliced in at both lights. Use the original wire to pull the new wire through the frame. this should provide power to the second light. At this time make sure the first and second light are grounded to a clean, rust, corrosion and paint free part of the trailer frame. here