jbird2212

So i have been searching for a while now and just cant seem to find the answer anywhere. The window mounted 3rd break light has 3 wires going to it, I know the ground is the black and I get power to the solid white wire. What is the white/black wire? I am trying to add a LED strobe to it as people do not seem to see me breaking and I see, to almost keep getting rear ended.

Thank you any help you guys can offer