Turn signal issue- '76 C-10

[Will Aygarn]

The front driver's side fender light & grille light works as running (parking) lights but they do not flash in turn signal mode.
The other lights work fine in running light, turn signal, & 4-way flasher mode, but the front driver side lights don't flash.
I tried swapping out the flasher units for G.P., but no change. Doesn't look like any of the fuses are blown.
Looking at having to trace the wires hand over hand, unless somebody can suggest where to look first.

 

[Charlie]

 

[1987 GMC Jimmy]

I suppose the chances are very slim, but have you checked the bulbs to make sure that one filament burned while the other one still works? A long shot, but I wanted to mention that. The wire that’d be causing you problems is gonna be that blue one. I’d be probing that as far back as I can to see where the wiring fault is. I don’t know if the turn signal switch in the column can fail in such a way as this. I’ve never heard of it, but someone else might know.