94 Chevy HD Cutaway right rear turn signal headaches

Hi!

I have a 94 Chevy HD Cutaway Van with wiring challenges.
I bought it very used and somewhat abused where the wiring is concerned, I am coming to find out. The ECM is for some reason under the driver's seat, the wiring under the dash is a rat's nest of connections, some of them covered in electrical tape, some just empty, none labeled. For the most part the truck runs great, especially for its age and I love it (it's also my tiny home) and I'm trying to better understand its parts and how to care for it properly. 

All three of the other signals work fine. I have tested and replaced the original right rear light assembly (it's a Grote 5 stud, in case that matters), thinking it might be that - it was so old and corroded I couldn't get any reading at all on a multimeter. I also added an LED strip to the existing trailer hitch connector and that works fine except for the right signal, which still does not light. I checked the ground at the rear of the truck, took it off, buffed the metal back to clean and reattached it. 

The back up lights also currently do not work in any of the three light assemblies at the rear and the backup beep doesn't work any more either - I am unsure whether this is a connected issue or a separate one but it might be worth mentioning. The hazard flashers also do not work.

So, given the other searches I've tried, I'm guessing it's got to be an issue further up the line - but I don't know where to look to find the connection under the dash for the right rear loop. My Haynes manual is no help, giving me no locations on my truck for any of its diagrams. The Haynes manual says that the right rear blinker circuit should be brown wires but they're actually green if the original connections on the old light assembly were correct.

I've trawled YouTube, reddit and many other mechanic sites. My own mechanic said he couldn't fix it.

I would so appreciate any ideas, and especially being directed to any photos or diagrams of the supposed location of this connector.

Without Quad Halogen Headlamps

 

Thank you but this is not what I was asking for. Pretty ironic given your tag line!
I already have this diagram in the Haynes manual I mentioned in my post. This does not show me what those connections look like in real life, nor where they are located (or likely to be located) - under the dash, in the central engine access, under the hood? on my truck.

The Haynes manual says that the right rear blinker circuit should be brown wires but they're actually green if the original connections on the old light assembly were correct.

This one must be different than the one you have.

have this diagram in the Haynes manual I mentioned in my post. This does not show me what those connections look like in real life, nor where they are located (or likely to be located) - under the dash, in the central engine access, under the hood?

It shows the wire for the right rear signal as green, just as you observed on the van. Unfortunately, most wiring diagrams do not explain the location of connectors or what they may look like.
Do you have power on the green wire with the right signal on?