Since you do not have a meter handy at the moment. When the dash glow light is on the interior light should be dimmer. As the glow relay switches off the light should brighten. If it remains the same through these steps there is no current of any consequence getting to the plugs.
Next possibility if the light does visually dim is you have a couple of glow plugs burnt out. For this you need at least a test light to place between the positive battery terminal and each separate glow plug wire.

Your car is a 79 I believe you mentioned. It may have the older glow plugs with the squiggly wires between the plugs. This is a horse of a different color. All those plugs are wired up in series like christmas tree bulbs. A quick test although not conclusive is you should see those squiggly resistance wires glow a slight red color in the dark if the system is working. This of course while the glow light on the dash is lit and towards the end of a reasonable cycle. Still the interior light will still dim when and if the glow plugs are on.

Or wet the end of your finger and lightly touch the squiggly wire about the same as you would do with a hot household clothes iron. There has to be some elevated temperature there even if the glow light only stays on ten seconds. Thats if the plugs are getting reasonable current. Remember those wires will be very, very hot. Test with your finger at the start of a cycle so the temperature is bearable. Not required if the car does the dimming interior light test during the glow plug cycle though.

There should also be a fuse present. If the sqiggly series plugs are in the car follow the #5 cylinder glow plug wire back to it. If the squigly wires are not present there might be a fuse in the relay box if it is out on the drivers inner fender. It may also still be out on the fiewall if the relay click is coming from around your knee area as it turns off. You can locate it by the click as it goes off. I have tried to keep this as simple as possible with no meter.

If you establish you have a problem you will probably need a meter to locate it. Splurge and bypass the 3.00 one in favour of perhaps a ten dollar one at the same source. Many other uses with time including around you business and home.
I just went back and reread your post. You own cars that span the two systems of glow plugs. The dividing line is 80 or 81 I believe. Even if pre 1981 the car could have had a semi updated system of glow plugs installed. If so there will be one continious wire from each glow plug to the next.