Is the glow plug relay a rebuildable/refurbishable unit on a '79 300D? If so, can anyone point me at a schematic or picture of it? I don't have that info in my FSM, and a search shows info on location, but no pictures of a unit apart.

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1985 300D-189k The 'UD', Ivory and Pinkamino
1979 300D-211k Dark Gray, Parchment
A 1980 Harley-~166k and
A 1994 Ford diesel pickup-349k and
A 1990 gasser Volvo wagon-145k