After the turn signal switch, the same wire, bulb filament, and ground path operates brake and turn.
So the turn signal switch is the fault and this fault is/was common to every vehicle that used the single 1157 bulb at rear. Nothing weird about it to some of us older service techs.
I see the little 3 inch square brake lamp lens on a 68 B'cuda as weird / far less than adequate for a passenger vehicle. A dang utility trailer has as much brake lamp.
I see a high mount 3rd brake lamp as a proactive addition to any classic, especially this example.
One more suggestion... Contact Daniel Stern Lighting and buy a quality signal switch. Those cheap ebay switches are crap. Good luck
