If you can't ohm the glow plugs like the other members suggested,
remove the glow plugs and test them using a jumper cables with direct 12 VDC,
ground to the glow plug body and positive to the tip where the wire screws in,
look for a red amber tip, keep fingers off hot plugs.
The ignition tumbler being loose, it's time for replacement do not delay,
or you will have a heck of time replacing it if the key doesn't turn.

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1986 190D 5 cyl. 5 speed manual White (215K),
1983 300SD Silver blue (103K)