A misunderstanding of series vs. parallel here.

Pencil glow plugs are a parallel circuit. All of the power ends are "common" from the relay and all of the ground ends are "common" with the block. The series plugs have the power from one plug series through the next, and the next, and the next, until the circuit is finally grounded to the block.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]