So the other day the glow plug light on the dash of the aforementioned '85 123 stopped lighting. The driver asked me to see about replacing the lamp since the car had started the night before, the first time she noticed it out, coming home from a restaurant (so the car had been up to temperature before cooling down for an hour or so). Perhaps unsurprisingly, the car was very difficult to get started the next morning, which pointed me to the thought that it's not the lamp that has failed in this case... :)
Anyhow, I've popped the cover off of the relay. The fuse is (or appears to be) good. Putting a voltmeter between the fuse and the block (the oil filter mount was in easy reach) showed me 12v with the key not in the ignition (this ruled out needing to find C105, which I couldn't easily locate in the guide).
So if the fuse is good and there's voltage there but the plugs don't glow, is it basically a given that the relay needs to be replaced? Another thought I had is that the temperature sensor might be stuck, but the drawing in the wiring diagram makes it look like it's all part of that same package, as a thermo-resistor deal of some sort.
thanks for the help,
-tom!