You may very well have some areas of very poor performing dielectric material, but I doubt you have a dead short. A dead short with the capacity of Diesel starting battery is going to make the bad part of a circuit board change phase a couple of times. Meaning, there would be a hole somewhere where all the smoke came out when the solid copper and other stuff became plasma. So, the fact that the system still works means the resistance path for the electricity is lower through the glow plugs than the other options it has. For now.

You should be able to measure the resistance to ground from the various points in the relay. Just follow the current path and find the weak links. Or just replace the relay module as it looks like it is on its last legs. Jim

__________________
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)