I fitted a led type third brake light on my classic. Due to the low voltage/wattage on led, I just tap it to the +ve side on one of the brake light (either left or right) and tap the -ve to one of the ground (cant really remember which ones, likely to be the ground wire from the towing electrics.
If you are fitting the bulb type third light, not sure if the extra wattage will foul the system, but you can try.