GP Relay / GPs work, Indicator Doesn't

[micalk]

Why won't the light come on!?

When I got the car about 8 months ago it wouldn't start "nicely" so I checked all the GPs, and with spares from my parts car I cobbled together five good glow plugs. Now the car starts like a dream. But my GP light on the dash never came on. Thought I had a bad bulb. Pulled the cluster and moved the brake bulb to the GP, and GP to the brake, and the bulbs checked good - still no GP light. Put 12V on pin #3 on the GP relay control connector and the GP light in the dash comes on. So I have a good bulb, a good cluster, and good wiring. What I don't have is a good indication that my GP system is working. The only thing left is the GP relay control unit. I swapped that out with my car parts spare and still no light. Admittedly they have different part numbers, but the wiring is the same so I would guess that they are interchangeable. When I pull up a parts supply website, the relay has the same P/N for both cars.

So I guess my question is, is it common for the current sensing portion of the relay to go bad, providing no indication of glow plug functionality? If one GP is marginal does the light stay out? What gives?