You need to trace all brake light conductors and their feed under the dash. Somewhere there is a short, bad connection, or chafed wires. You know the harness from kick panel to rear of car is good, so the problem is between fuse block, brake light switch, connections to directional signal switch, and connector in kick panel.
Make a copy of wire diagram from here, clip it on a clip board, and systematically trace it all out one wire at a time in the car marking each branch of circuits tested on the diagram as you go. This method helps keep one on task, and prevents missing something that may not be obvious when pawing around under the dash.
You will find a great bundle of wire looping up and over steering column just behind face of dash where wires connect back to fuse block. Look these over carefully along with light buzzer circuit.
