I think there used to be a big fuse in the box too. On the older cars this is a chunk of metal with holes that some screws go through. After a lot of serious heat cycles it buckles and then begins to fatigue. Eventually it cracks and you don't get juice to the glow plugs. Since the ends are bolted down, it doesn't move when it fails and you may have some difficulty seeing that it is cracked. Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)