The older engines use series glow plugs, so you could check for wiring (including the zig-zag resistance wire in some places) between the glow plugs. The later engines used parallel glow plugs and had an individual wire from the glow plug relay (on the driver's fender) to each glow plug.

This is subject to an owner upgrading the series style to the parallel style.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles