1983 300 TD
So every glow plug has .06 ohms (4 of 5 are new)
They each get 12V from the harness
The glow plug light never comes on, BUT, when you unplug the harness the glow plug light goes on (so I know the bulb is good)

So the glow plugs work but the light doesn't come on and if the harness is unplugged then the light comes on.

Any answers?

I would suspect a problem with the GP relay, which controls the light as well as the GPs.