I'm not sure of the exact sequence, it is different for different models, but I think that if one plug is out, the lamp will flash after you start, if two are out, the lamp doesn't light, but flashes after you start, and if more than two are out, never comes on at all. Distinct one second or so on-off sequence, not dull glow.

I would suspect you have a loose wire on the ignition switch or on the glowplug relay, so it gets false on/off signals, or a bad ground in the dash so that you have current backfeeds. I had a bad ground on my Volvo headlight, left side low beam -- caused the dash lights and gas guage to "flash" with the turn signal, all four lamps were on, very dim, on low beams, and would go out with the flasher. Put the ground wire on a good terminal, entire problem vanished. Weird.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!