Are you familiar with the terminology “voltage drop test” . . . ?
If so, you need to perform a voltage drop test to determine where you’re losing available voltage, as you already stated
for all I know, you might have significant voltage drop through a connector . . . in other words, 14V on one side of a connector, but significantly less on the other side
here’s a silly question . . . you are actually turning the headlight switch to the on position, not just relying on the drl to do its thing. I specifically mention drl because, as far as I’m aware, this truck never had auto headlights, just drl. I’ve seen situations where a guy would start the truck with the headlight switch in the off position, the headlights would come on dimly . . . until he actually turned the headlight switch to the on position. On some vehicles, drl alone doesn’t provide full voltage to the headlights
I’ll try to help as much as I can . . .
go to gmupfitter.com
now go to body builder manual
on the right side of the screen, choose medium/heavy duty vehicles, specifically, “click here for model year selection”
Once you’ve done that, you have more choices
Choose “electrical medium duty C-series” . . . it’s a pdf
It’s not perfect, but it has component locations, pinouts, schematics, and so forth
maybe it’ll give you an idea where you need to look for that “missing voltage”
another thing . . . exactly how are you measuring available voltage to the sealed beam?
Directly at the sealed beam 3-pin connector, or somewhere further back, teed in, for example?
Are you sure the sealed beam ground is okay?
Have you hooked up your meter positive to the positive terminal for the sealed beam, and hooked up the meter negative to another ground, for example battery negative post?
If you now see 14V at idle, then your sealed beam ground has a problem
Please forgive me if you’ve already done all this or if it sounds as if I’m talking down to you. I’m just throwing some ideas out there