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Thanks for all the help so far.

Grounds are good I believe, I have a separate ground wire for the taillight assemblies that fixed a problem a couple years ago.

When I hook up to the wire that feeds the headlight source the lights all work, which is why I think the grounds are okay in this case.

I agree with a couple of posters, I will have to start cleaning contacts, probably start with the fuses, although I have removed and reinstalled them which will frequently improve the contact enough for a circuit to work.

This portion of the harness is all original so as mentioned maybe the feed wire itself is bad and will show 12 volts with a multimeter but as soon as a load is on it the resistance is too high (this could also apply to a connection of course).

It really is amazing the gauge of wire the factories used when these were new.

I agree grounds (or the lack of) can cause some baffling problems so I will explore that but I think in this case it must be on the feed (hot) side but ....

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Could be a combination of both, electrical circuits ARE tricky. However the feed wire may have an open spot/bad connection or "fracture" due to age/corrosion.