Troubleshooting glow plug issues

[Oilburner IDI]

I have a 1979 300SD that was not treated well in its past life. The previous owner had a penchant for Mickey-Mousing repairs to the car. I don't know what all has been done, but something's wrong with the glow plug system. Regardless of engine temperature or outside temperature, the glow plug light in the dash would only stay on for 2 seconds or so, and go out and not re-light. I had figured that it was a wiring issue and that what was supposed to be the glow plug light was actually the seatbelt nag light, but now I'm thinking something else.

The car had had difficulty starting over the last couple of weeks, so I tested the glow plugs today. 4 were good, and 1 was bad...but, that was not all I found. It looks like I have at least three different brands; at least one Champion, and I think one Bosch, and I couldn't readily identify the others (although I have not removed them). Also, after reading part of the W116 shop manual (on pdf), it looks like I have two different TYPES of plugs. According to the manual, there are "standard" and "fast-start" glow plugs, with the external difference being an aluminum collar on the "standard" plug and a brass collar on the "fast-start" plug. It apparently takes 9 seconds for a "fast-start" plug to get to temperature, as compared to 20 seconds for a "standard" plug. I believe I have two brass-collar plugs (including the Bosch) and three aluminum-collar plugs (including the Champion).

I have a feeling that mis-matching these plugs is not a good thing for the system, but the question now is, which type should I use? At this point, I plan to just replace them all. It looks like the relay is different between the two (from what I can see, the "standard" setup was used in '78 and '79, and the "fast-start" setup was used in 1980), but I'm not sure which relay I have, considering the history of this car. Also, what is the glow plug light in the dash trying to tell me? I thought it was supposed to flash if there was a fault; could I be seeing a bad relay, or could this odd behavior be the result of mixed plug types?

Thank you for any insight BTW, the engine was replaced at some point in this car's life; I'm not sure what model year the replacement engine came from, though. The previous owner thought it came out of an '82 300SD, but it has non-EGR manifolds. I wasn't able to get any more information from him about that. So, it's possible that parts of the other car's GP system were brought over to this car as well.