i had all of the glow plugs replaced in the winter and the light is working like it should, thats good enough proof that they work for me. the glow plug light comes on usually when even only one of them is out.
the fuel line idea does make sense though. i ran diesel purge through my engine a while ago so i had to disconnect the fuel line. perhaps it has come a little loose.

thanks alot, thats the first thing ill check.

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1991 300 D 2.5 Turbo, 220k
also in the family:
1981 240 D 185k
1991 350 SD 185k
2006 S 500
2005 SLK 350