It sounds kinda like a broken wire to me.

You can divide and conquer the problem to some extent. Measure for voltage at one of the non-functional light bulb sockets. If you have voltage there, its a bad ground. No voltage, its a bad 12V supply wire.

So at that point you've divided the problem in half, and can then trace in the proper direction. In difficult cases the factory wiring diagram helps because it shows wiring junction points.

Ken300D

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