Excellent idea! I'll check it out and post back.

It was obvious which one was ground, and two out of the three remaining pins gave electrical current when supposed to (turn signal, daylights). Fourth wire was dead.

The BRN wire is ground.

Did you have someone step on the brake pedal when you performed this test? Current is only present if the brakes are actuated.

Yes, I did. This issue is still unresolved and I strongly suspect the brake light wire (somewhere) to be damaged... :(

There are tools available that will allow you to trace a wire to determine the location of the break. It sends a tone signal through the wire and you have a receiver that can pickup the tone. If you are not a professional mechanic or not a hobbyist car guy, it would probably be cheaper to take it in and have a technician locate and repair the broken wire rather than spend the money on the tool.

https://www.youtube.com/watch?v=Y0HjaR0tj7Q

There is A common issue of the wires in the rubber boot between the body and the tail gate being pinched and fraying. Pull off the boot (actually pull it down) and check to see the condition the wires are in.
This maybe the source of the issues.

Common on those vehicles where the trunk is opened and closed a LOT.

I thought the brake lights were located in the rear quarter panel of the car and not the tailgate. Only lights that I am aware of in the tailgate is the backup lights and if you have it the rear fog lights.