In the early 50's there were still lots of vehicles that were wired with a separate brake light circuit. Those became obsolete as more and more car makers adopted the dual-filament combined brake and turn signal wiring method.
The original brake controllers also had a separate brake light wire so that when you manually activated the brakes, the trailer brakes would light up to warn following vehicles.
Nowdays with modern brake controllers and pre-wired connectors for most tow vehicles, there is no need for the separate wire. If you manually activate the trailer brakes on your Ford, Chevy, Dodge or Toyota, it automatically light up the appropriate lamps in both your tow vehicle and trailer.
So if you want to you can re-purpose the original black wire in your 6-way harness as either a 12V
charge line (early Airstreams had a 12V
charge and ground line that was separate from the 6-way) or you can add back-up lights. Be sure to use appropriate wire sizes and fuses or circuit breakers.
The 99 had two bulbs. One was a single filament powered by the running light circuit, the other was a dual filament powered by the turn signal and the brake signal.
You can leave the running lights hooked up to the lower single filament bulb, and just combine both wires from the dual filament bulb to the single combined brake/turn signal wire from your 7-way harness.
Or you could convert the tail-lights to a LED type brake/turn signal. There are several threads that discuss how to do this.
Here is the original wiring schematic.