I decided to finally tackle the problem of the rear side marker lights not illuminating. While not a big deal as I don't drive the truck at night often, it is something that needs to work.
The lens was cracked and chrome missing on the driver side marker light, so I replaced that with a new OEM assembly. With how shiny and chrome it is, it looks out of place on the truck.
Then I chased down why the lights were not getting enough voltage. After tracing back through the damned wiring harness mods the current owner did when he was in college, I reconnected everything and got the driver side light illuminated. The passenger side light had the grounding wire break off where it was welded to the housing.
Thankfully I had kept the original driver side housing from 1973. The driver side and passenger side metal housings are identical down to the part number. So I took the rubber gasket, lens and chrome trim from the passenger side assembly cleaned them and reinstalled the lamp. What do you know, the rear marker lights are now working.
But of course fixing one problem leads to the discovery of another. When the running lights are on and I activate the driver side turn signal (via the turn signal lever or the hazard switch) the driver side brake light blinked instead of the turn signal. So I pulled the lens off (and the original rubber gasket disintegrated) to see exactly what was happening. Looks like a bad ground as the turn signal is weakly illuminating, the rear marker light is brightening and the reverse light is weakly illuminating.
The way I see it I can either:
1) Add a grounding wire to the housing (likely using one of the two mounting screws) and ground to the frame. I'll need to get a new gasket. And since I can be a bit obsessive, probably a new lens since the original is fogged up. Which would lead to gaskets and lenses on both sides.
2) Replace the assembly with an OEM assembly. This would require changing the connector to a flat four trailer connector (because of that darn current owners mods from 14 years ago). This will likely require the addition of a grounding wire.
3) Or upgrade to these Model 42 - Products - Narva . But those are sealed units without replaceable LEDs.
4) Or none of the above (see next post)
I'll probably start with option 1 as I can implement that sooner than later, and delay the upgrade decision.
EDIT:
I tried a ground wire from several locations on the turn signal housing to several locations on the frame and the problem persisted. My guess is this is shorting somewhere in the harness...now the question is where.