I am not sure you understand the glow position. Let me describe the four positions of the fuel shut-off/glow switch.
The first position is off; with the knob pushed all the way in. The second position is the run position. This is the position the knob takes after you have pulled it and released it. The third is the glow position. This position can not be kept without physically holding the knob against the first spring. The best way to tell if this position is engaged is to watch the lights on the dash. When the switch is engaged and the glow is functioning the dash warning lights will dim from the load of the operating glow system (same as four sets of headlights). The fourth position is start. It is a small move against the last spring to pull the knob all the way to the start position.
The glow indicator is not a light it is a heating element. The 190Db I drove for 5 years had an element that was hard to see and I just would put my thumb over the grill and start when I could feel the heat (shortly before it would burn me). If you have a glow system that works properly it should glow that element bright red if you hold it long enough. The curved bars between the glow plugs will also glow red if the system is working and held long enough.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician