95 e 300d 118k. Glow plug light would come on randomly - at speed, at start up, whenever. Sometimes it wouln't come on at all. Obviously I suspected glow plug problem, carbon fouling or relay. My tech checked it all out and discovered engine wiring harness was cracked in multiple places. Wires were even showing and starting to fray. He said MB switched from using an indestructable wiring harness on the earlier models to using a cheap version. Even the new MB replacement is low(er) quality. He replaced engine wiring harness and now things are fine. Light acts as it should. Cost me $700. Still kinda stings.