Every once in a while something on my car will fix itself. That seems to have happened today.
For years I've been waiting to fix a left front turn signal problem. So today I have the bumper cover off and the light fixtures out to possibly replace the bulb. Inside the Civic's side marker light bulb are 2 filaments. With the light bulb plugged in and out of the headlight fixture I tested it again. The directional filament would not light up under any conditions and caused the parking light filament to intermittently shut off whenever I turned on the left turn signal. Hazards also worked improperly.
I did a "known good" test by swapping the left front turn signal bulb with the right front turn signal bulb. I didn't do anything else. I didn't clean anything or detach and reattach anything other than the bulbs. Suddenly both light bulbs worked normally. Then I put them back in their original places. And the bulb that has been failing consistently for two or three years of daily diving started working completely normally.
I am guessing that in removing the bulb I cleared some kind of minor corrosion or dust/dirt (the bulb housing was coated in fine road dust).
I suppose it might also be an intermittent open in the wiring.
Or a ghost in the machine.