I tend to agree with Hasenwerk on the after-glow of the engine starting GPs. My only experience that leads me to agree was working on friends 01 Jetta with ALH engine about a year ago (two-wire set-up). I opened the GP Relay. Using temporary wiring leads, by observation, I did see that the main contacts in the Rely that provide power to the GPs did not close (ambient in the 70s, Fahrenheit). And, there is circuitry in the GP relay that allows GP condition monitoring for an instant when the ignition is turned on without glowing the plugs, regardless of engine or ambient temperature. It's all right there inside the GP Relay.
So, how could there be after-glow if there was no glow?
The GP system in the MK1s keeps the GPs hot for a few seconds after the indicator light goes off. Back in the day when my hearing was good, I could hear the relay click off. Also, observing the aftermarket volt gauge indicated that voltage bounced back when the GP relay kicked out.
This discussion has enticed me to find that GP Relay in my stash of parts and do the test and observations again.