Originally there were two wire cords coming out of the tongue, one on each side. They were made to plug into special "tap-a-light" bulbs that would replace the original car bulbs. Those wire cords were cracked with bare wire exposed on our Uhaul
so I pulled it all out. To use a standard 4 wire plug, I got a long cord with a molded on 4 prong plug from etrailer. I ran the cord along the tongue and up through the floor into the storage area just to the rear of the closet. At that point I installed a junction box. From that box I ran new wire to each tail light
for turn / stop / tail, and ground. Note that the original tail lights
had two bulb sockets. The lower socket is for the red part of the lens, which is tail and stop lights. The top socket is also for a dual filament bulb, but only one filament is used for the turn signals (amber). The other wire is not connected. I only used the red part of the lens until installed a set of the all red lenses, which are no longer available. The wires for the marker lights drop down between the walls and are glued in spots to the outer shell so I left them and connected them into the junction box tail light
wire. The trouble spots are the grounds, both at the marker lights where the ground wire is just under one of the screws. Those marker lights can be the source of water leaks
. Another ground issue that took me a while to figure out was the tail light
bulb socket. First get a socket brush and clean the sockets and contacts. Make sure the contacts don't get out of position. Then see that the bulbs fit tight in the sockets, this is where I had a ground issue. To tighten the bulb in the socket, at the slot in the socket, bend it in slightly at the point where the pin on the bulb turns in to lock the bulb in place. If you want to use the amber part of the lens you will need a converter for that, also from etrailer and other sources.These old trailers seem to be prone to modifications, some of them obviously done by butchers, so you never know what was original or modified. Tail light options is a whole other discussion.