I’ve looked high and low for the main lighting plug for the rear of my 1986 f-150 no junk yards around me have it and I need one badly anyone have an idea of where I can get a factory one or even an new plug style I can switch out

 

Here is a universal kit with the special crimp tool included.

 

Are you talking the tail light harness or the frame rail harness & tail light harness?
If it is the tail light harness do you need the light sockets, wire and the plug for the rail harness?

Lets say you need everything from the plug to the sockets.
You can get the sockets from any parts store both the tail and back up sockets.

For plug and wire I would look at the cheap trailer light kits.
The plug may not be 100% right but I believe it will work but if not I know NAPA has different plugs from 1 to think 7 and I am sure they have one that matches the rail harness.
Wire will be long enough to reach to the right side tail light housing.

I would start at the plug and get that worked out and then you can "ping" (test) the wires to see where they need to go.
A little solder and heat srink or butt connectors that have the srink on them to make them weather tight.
You would have to wire in the plate light(s) but should not be hard.

It is not that hard to make a tail light harness as I had to make one for my flare side and I used the style side parts truck harness and the plate light harness is different on a flare.
Dave ----