If using prewired lights, run a pull wire up from the rear end of the chassis rails up through the gap between the front chassis rail and the drawbar. Secure the cable and pull the cable through chassis rails leaving a little extra cable to allow fitting and removal of the light unit.
Thread the pull wire into the cable hole drilled in the drawbar and feed it up to the coupling end of the drawbar. Again carefully pull the cable through the drawbar.
Once through, manually thread the cable up and out of the slotted hole in the top of the drawbar.
Repeat the process with the other light cable.
Use grommets where possible to prevent chafing and damage to the cable insulation coating. If grommets are not available, use a good neutral cure silicon caulk around the hole and cable(s).
Bind the two cables together with insulation tape where they exit the drawbar and slide a heat shrink tube or flexible conduit over the cables to enhance its protection.
Cut the cables to length ensuring there is adequate length to allow full motion of the trailer without kinking or stretching the cables (do not let them droop onto or near the ground when connected to the tow vehicle), strip the ends and wire into the plug.
Test the lights and when everything is working correctly, mount the lights to the trailer.