2.5 na doesn't have a relay, glows work directly off the ign switch.
Thick wire with large terminal on rear of ign switch goes to number 4 glowplug.all this from memory BTW
Edit to say- I think a bit more memory is returning.... There are 2 thick wires with larger terminals on the rear of the ign switch, one will have 12v the other will not UNTIL you turn the ign key to the glow position when the other thick cable will be joined to it (internally of the switch, you can't see it!) and send the 12v to number 4 glowplug. glowplugs are a heavy electrical load and the wiring/ switch do suffer and fail. you can join the 2 thick cables together to get the glows working as a test to see if the switch is faulty if you don't have test equipment but don't let them cables arc on any metal! you can buy the ign switch terminal block on its own, quite cheap as well iirc, its held on with 2 I think, little screws.
take the coloum shrouds off to acess the rear of the switch maybe the clocks too can't remember.
the Gubbins in the battery box are not standard so could be anything.
mine has the same glowplug circuit as yours BUT I've fitted a relay to take the strain off the switch, that could have been done to yours? Who knows what a previous owner has done..
First test is for 12v at number 4 glow, then work from there.