Not all trailers come with an outside cable connection. I had one of those trailers at one time. I just bought a flat cable that I could run through the window and it worked just fine for me.
That being said, some manufacturers put the connectors are under the edge of the trailer on the side by the dump valves. Just look around under the edge for that. It would just be a simple cable end. Other than getting dirty from the road, it should work fine if you find one.
Laura