It's ALMOST impossible for them to fail in a "PARTIAL" mode.
you either get Voltage @ the Pins or you do not.

You might be able to make a case for "Dirty" contactors inside the Relay,
Some have taken the cover off and gently cleaned the "Dirty" contactors
with superfine Emery.

The One GP that measures 18 Ohm is Really Bad.

When you're measuring the Voltage to the Glow Plug Harness Relay Pins ,is the
Harness plug disconnected from the ReLay?
If the Harness is still plugged into the ReLay, that 18 Ohm GP could be pulling
down the System.

What's the Voltage @ the Power supply connector (The RED ,HOT @ all times)
cable at the Glow relay when the system is in active "Glow".
That 12.9 volts with the Key in your hand is nice...BUT tells you nothing
about what's going on in operation.

I'm still hesitant to condemn your GP relay.

[connections,and grounds grounds grounds]

__________________
'84 300SD sold
124.128