Those front wiring harness a brutal for falling apart inside the wrapping and causing all kinds of fun !

I'm still banned from climbing in the loft until I heal but I have some pretty good examples of casings getting brittle and arcing over to another circuit.

I apologize for the heavy stepping in the video.

If you noticed, I have to correct the colour of those front bulbs 

I've been thinking about this, and think it's a ground problem. When I bought my 72 GMC the signal lights were acting up intermittently. Then I noticed the rear marker lights were flashing. I checked with another user, and while it's correct in that application for the front marker lights to flash with the signal lights, it's not correct for the rears.
I ran a separate ground wire to rack housing to no avail. It them occurred to me the truck box has an intermittent ground. Running a new ground wire to the box solved the problem.

So, triple check all the grounds, ensuring good connectivity all the way back to the battery - post. And not just to the bulb sockets, make sure the bulbs in the sockets are seeing a good ground.
I believe what's happening is your signal lights are seeing a ground through the filament of your marker lights.

Just last night I may have resolved this issue. Gonna confirm shortly. I traced every wire for feed and continuity. Stumbled upon lh side rear yellow and brown wires both having joined continuity. Traced the issue down to socket terminals slightly twisted and arcing together. Adjusted them and solved the joint continuity. Too many things ripped apart still and too late last night to confirm but I will definitely know today. Will keep you posted. Thanks for following up

Excellent troubleshooting, glad it all worked out.