That glow plug light ALWAYS acted that way. A quick flash is all you get, regardless of glow plug condition. The relay is on, just not the light.
You just need to check their resistance, and/or pull them out, supply 12 volts DC to them and watch for a really completely glowed tip. You can visually tell the difference.
They were all working recently, but it has been a really long time since I'd had them out, so maybe one is working, just weak.

__________________
Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

Gone:
'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
'80 240D 154K "China" Scar engine installed
'81 300TD 240K "Smash"
'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John