When you say the light comes on, is it flashing or on steady? I recently had my glowplug light flash after the engine was running. It turned out to be a loose wire on the #1 glowplug. I don't know if the newer glowplug systems had this diagnostic feature.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member