You have series glow plugs so basically they are most likely okay. Sounds like the relay does no longer reliabily turn on. In just my opinion would try another relay first. Could be an oxidized connection or even a failing key switch as well though but again the old tired relay would be a suspect first. One other thing I should mention is once in awhile one of my cars the 77 300d does exactly the same thing. I just turn key on and off a couple of times till light comes on. I have never troubleshot this problem but if car was in daily use would always keep a jumper in trunk so in the event of total failure would bridge positive battery terminal to top of glow plug chain to get vehicle started. The older relays are not in demand as much as the newer style for parellel plugs so a junk yard replacement is a possibility. A search of the archives is in order as well as the problem is pretty common I think. (intermittent glow plug function) (the relay is located above drivers feet is silver in colour and has a machine screw and nut to fasten it to pedal bracket support if you do not have location information as unlike the newer and different ones it is not in plain sight. I only threw this in as your service information may be limited)