1972 D100.
The brake lights work OK during the day, but the left one doesn't work when the lights are on at night.
Everything else works normally.
Had the owner wiggle the turn signal level last night to check the turn signal switch, with no change.
I guess it still could be a really burnt contact on it, but I am just guessing based on some posts I've read.

I have no idea why a brake light on one side would stop working when the headlights are turned on.

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63 Signet / 10 inch front drums / weber 2bbl / HEI / 2.76 rear end

Bad ground at the tail light socket, or bulb.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

Yup, Charrlie's probably nailed it. Screwed-up bulb or poor ground at socket.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

So embarrassing.
I should have known it was a bad ground.

I fixed it.

Any explanation on why the left brake light stopped working with the headlights and taillights turned on?

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63 Signet / 10 inch front drums / weber 2bbl / HEI / 2.76 rear end

Because the brake light filament had been grounding through the park/tail circuit, and once that was turned on it was at +12v relative to ground. Since the brake light was also at +12v relative to ground (when you were stepping on the brake) there was no voltage across the brake light filament, which therefore did not light up.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.