Marker Lights aren't working on '03 Freightliner FL60

2003 Freightliner FL60 Dump Truck.
When turning on the lights, all the marker lights on the truck turn on and then cut off after 1 second. The brake lights, signals, hazards, and headlights all work Normally. Only the marker/clearance light are having issues.
The problem occurred while I was driving (Jan. 2019), everything was fine to this point, then out of nowhere all the marker lights cut out.
After the first time the cut out, when you try again, only the front marker lights turn on, while the rest of the truck is dark, then the lights cut out after a second.
I also had the lights flicker by them selves, even with the switch in the OFF position. The lights would flash once every 10-15 seconds.
When the lights cut out, you can hear the relay click. I swapped out the relay and still had same problem. People suggest corrosion, but I replaced front markers (pedestal light on fender) and tail lights with LED (July 2018). All those connections are tight. All marker lights on the dump box were not replaced, visual inspection on those seem to check out.

 

I didn't notice any damaged or corroded wires. I will look again incase I missed something, Figure it could be something inside the lights themselves? However the marker/brake lights and signal lights are emitted by the same light box, and the signals work fine.
Ill see if I can find a short somewhere in the system. and send an update.

 

Yeah, that raises a few more questions.
The system on this truck has one single, 3-position switch. OFF// All Marker, Clearance and Dash Lights // All Markers PLUS Head Lights. All the lights have their own Relay; Head Light, Marker Light, Left Signal, Right Signal, Brakes. Etc all are on their own relay.

I figure that the front marker lights only come on because my Short is in the back of the truck somewhere.
I kind of traced the wires and it basically goes from the switch/fuse, to the Cab clearance lights, then to the marker lights in the front (Newly installed lights) then from there it goes all the way back to the tail lights [(Newly installed Lights) Tail Marker lights same problem, brake light works fine] then wiring goes to the dump box. So I assume the short is in the back causing power to not get to the back half of the truck at all. So that just the front marker lights light up. With that said though i still haven't found and damaged wires.

 

That'll be difficult, as the side marker lights and tail lights are LED so I would have to completely cut the wire, rather than just pull the bulb. And its a balmy -27C today.

 

Update: I disconnected every light, one by one; and had multiple lights disconnected simultaneously. The problem continued throughout the entire diagnostic. I visually inspected all wires/harnesses that I had access to. As well as; at all connection points, including connections to the trailer harness and wiring at the box's pivot point. All wires seem to be in good order. No rubs, breaks, burns, or corrosion. I did not check wiring going along frame rail as they are all inside of a sleeve.
I was told by a mechanic friend that it could be a problem within the trucks computer (but that seems unlikely to me) or within the fuse/relay panel somewhere.

I have a feeling this is going to be a very expensive and time consuming issue...