3rd Brake Lamp (CHMSL)

That is a slick custom addition from some other vehicle. Maybe bodywork before paint grafting.
My high mount brake lamp is inside on the package shelf. I had to cut its base into 3 pieces and mod its angle to get it to hug up to the rear glass of a notch back. @slantsixdan can hook you up with an easier adapted fixture design if you want to go this route.
One point to note; These older cars use the same wires for both brake and turn. My aftermarket fixture came with an electronic box they called logic circuit that prevented the added brake lamp blinking with turn signal. It died after about 2 years. I went and bought 25 feet of white wire and routed it from brake lamp switch at pedal. Same path as OEM rear lamp harness all the way to the added brake lamp. Problem solved forever. I did use a scotch lock wiretap to attach the added wire. No way I would cut that wire and risk losing all brake lamps.