You have a couple of important issues to address here.
First, forget about the glow plugs, and everything else on the car, until after you get the temperature gauge fixed or install some kind of alternative. Don't drive it without a gauge, you need to know the temperature and be able to react if it goes outside normal range. This engine is sensitive to overheating, if you have no way of knowing about it until you notice operating symptoms, then you're guaranteeing that even a very small cooling system problem will cascade to a complete engine failure. You'd hate to lose the car that way and it is avoidable. Fix this first, before you drive it another mile.
Then, to address the glow plugs, we need to know a little more about what you're working on. 240 or 740, and what year? 240s up through the first half of model year 1984 used a glow control system with two relays then from 1984.5 they used a single relay system. 740/760 has the single relay system for all years. It sounds as if you may have a relay issue, or a wiring problem that has not been found yet. The temp sensor in the head rarely fails. Does the light *never* turn off with key in II, or does it just take a long long time? Do the glow plugs remain energized the entire time?
If the car came to you with all 6 plugs dead, that's a good sign that there is a glow system problem. So yes, you should be concerned -- that will happen again if it remains this way and you'll have to get acquainted with that #6 plug again.
After you fix the temperature gauge, provide some more info on the car and the glow issue and we'll get it figured out. The system is simple and should be able to get put right without too much trouble.
