glow plug light comes on after starting, hard cold start

[starshipcomando]

1999 350SDL. Cold weather starts were not a problem up to this point with normal glow plug light cycling between on and off. Then suddenly the glow plug light wouldn't come on and cold starting was real hard with many attempts at cycling the glow plug light. To get the glow plug to light takes several minutes in the correct ignition key position for glow indicator light coming on. Even after waiting a couple minutes for the glow plug indicator light to cycle from from lit to out (indicating you should crank starter) - starting requires cranking for extended periods. It actually has a better chance of starting if I don't wait for the glow plug to light. So what's your guess? Bad glow plugs? On my 1986 300SDL... I had one bad glow plug and yet the glow indicator light still functioned properly. That makes me think it is not bad glow plugs, but something else. BTW - warm starts are puuurfect especially after plugging before engine gets cold - which is not very convenient.