As I see the drawing the connection to the lights is several different circuits and then connect6ed to the different circuits of the Ford wiring.
The drawing shows the connection tot he Ford wiring being up front with the Winn wiring coming to the rear following the top edge where wall meets side. Then the group drops down the left corner to the left set of lights.
If we think of how the stripped chassis built by Ford and then the RV cabin part built on top, it makes sense. Since Winn is only extending and not adding any more circuits, they are prone to just connecting to the FORD wiring at front and running wires to extend to the back as they build the RV parts.
What that says to me is there is no separate fuse for the taillights in the trailer connection that is different from the taillights on the coach!
Basic idea is that if we get power to the taillights on the coach, we only need to look at why it is not getting on down to the trailer connection!
I don't feel there will be a different fuse for the trailer connection taillights from the normal one for the coach mounted lights.
They almost have to be the same circuit if we turn them both on at the same switch. If they were different feeds, we would have to have two switches, one to turn the lights on for the coach and a second to turn on the trailer lights! One switch turns them both on, so we have to have only one power feed fuse!
If power is good to the left coach light, power is good to that point. Then if it is good to the right, we know the connection that extends across the back is good, also.
But the power is not getting from the left taillight on down to the trailer connection. If we had some work done, etc., a wire might have been cut.
But the most likely thing is corrosion at a connection!
Since they do show there is a plug along the path from left taillight down to the trailer connection, I would have to really look hard for where I'm missing that plug!
Logic says they would leave it out for work and not hidden in the body.
That's trust that may be suspect, though!
But I also know what you mean on not being able to see something like a plug if you have a bunch of wires and maybe covered with some form of cover like a wire loom or a wad of tape!
A bunch of wires, with a bunch of "something" covering them and then throw in a bunch of dirt, road oil, etc. and it may be right there and just look like a funny lump!
It certainly "could" be up where the left light is tied in but when they show there is a connector plug down near the bumper, I'm a hard head to think it is there and hiding somewhere close!
Not much help there but just what I think I would look for! I would have to start at the plug and take off the cover over any lump that might hide a small 2 pin plug! They kind of have a habit of putting some form of plastic tie or straps right at where the two ends plug together. that helps to keep the two plugs from pulling apart as the wires flop up and down.
But it also makes us less likely to spot the plug because we would have to cut the straps to see it under the lump they make! I sometimes find I am against taking things like that apart because I then have to replace or repair it.
But that can also make me miss what I'm looking for!
Beware "saving time" can also be shooting our own foot!
Sorry, setting here at the keyboard is lots easier that crawling around under the RV!