Off the top of my head, I think that is the wire for the tach feed. I think there is a six volt wire, a twelve volt wire, and a tach wire feeding thru the ballast. I haven't used the stock stuff since my first TR8 20 years ago. The one I am restoring now still has it, but that's just because I haven't got around to ripping it out and replacing it yet. My crystal ball shows a Mallory Unilite distributor, coil, and hyfire ignition box.