This type of problem is almost always a bad signal switch. The brake light switch power feeds INTO the signal switch and the signal switch then routes the power to both turn signal lamps, depending on whether the switch is centered or on one turn signal.
So if the signals work properly, this means the bulbs and harness/ wiring is OK, and if one brake light works, this means the brake light switch is OK and getting power to the TS switch.
On the newer cars with the 4X flasher switch integral in the TS switch, there used to be a popular failure involving that part of the switch. "Back then" I ....."fixed".... a few poor sailer's cars temporarily by cutting off the 4x flasher knob (no longer used) and re--routing one or two wires in the signal switch. It got them "on the road" with little more than some time and a non--functional 4x flasher