Probably covered before but here it goes....

-64 Dart one side brake light and turn signal not working.
-Running lights were fine (it's a dual filament bulb)
-First thought was bulb.

Discovered Bulb was good.

-Second thought was the turn signal switch since as others have pointed out power runs through it.

But.... my experience with my other car was the switch contacts don't fail all of a sudden, they become
jiggly/ intermittent .

Turned out to be easy peasy. Unplugged the connector under the dash & WD40 cleaned contacts.
I knew I had licked when the click - click flasher tempo got faster .... the bi-metallic element had the load
of both front and rear turn signal.

Thanks for sharing the fix. We sometimes look for the most difficult theories, when simple remedial cleaning of corrosion is the best answer.

_________________
"Louise", a 1976 Dart Custom project, (now sadly reverted to being just an "organ donor" to our other project Darts.)