What's the Problem?
You've Diagnosed it Already!

"When energized(key in preheat) I get nothing 0 V at the gp nut on #1 or #2 (didn't check further). The pins with connector off are getting 11 V."

Resistance needs to be checked at each individual Glow Plug ...
Not "Between Them"

I'm not sure you're not having a "Semantics Problem" describing your checking
of the Glow Plugs???

If you have Proper Voltage at the Relay Pins and Improper at the GPs ...
It's The Wiring!
(Or Alien Abduction of the actual Copper wire inside of the insulation of the
wires from the relay Pins to the Glow Plugs.So you can't figure it out and
the Aliens get to enjoy driving you crazy.)
EDIT: The Aliens might be "Sucking the Electricity" out of your GP wires
whenever they see you testing...Hard to catch the little boogers!

Take each individual glow plug wire and test it for continuity with your DMM.

Then "Ohm Out" each wire (From one end to the other,with both ends
disconnected from the Car.) There's a formula online given the AWG or mm
of the wire and it's length for proper resistance figures.

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'84 300SD sold
124.128