Try leaving the key in the glow position even after the light has turned off. Like SD Blue said you will hear a click and the dash lights will brighten after the plugs complete their 30 second cycle.

Correct me if I am wrong but I thought if the glow plug light operates the plugs are alright?

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine