This is how it works:



nr. 2 is the pull knob
nr. 3 is the reed contact for the glow light on the dashboard (also shown on the picture above)
nr. 4 is the glow plug light itself
nr. 6 is the thermotime switch, it breaks the circuit of the glow plug light after a period of time, depending on engine temperature, indicating readiness to start the engine.

nr. 1 is the ignition lock, nr. 5 the glowers.

You don't really need a relay if you upgrade, simply put in pencil glowers, connect them with heavy gauge wires, disconnect the ground strap, rewire the glow plug light around the thermotime switch (it could perhaps work with the thermotime switch, but it will show the readiness to start for old glowers, typically 30 seconds for a cold start). The current draw might not be high enough for the reed contact to close, but I don't think that is the case. The proof of the pudding will be in the eating.

You can use a relay, but it requires a bit more rewiring, depending on the relay.

The relay can indicate ready to start and it can afterglow, i.e. let the glowers glow for a short time after the engine started, but it depends on the type of relay.