Weird Electrical/Brake Question

So, as I replaced the wire from my battery post to the starter, I noticed a dangling black wire in my 73 Duster, with a sort of right angle bend cap on the end. As I looked down where it was dangling, i saw a metal block with brake lines running in and out of it (distribution block?) and it looks like there is a little metal nub on there that this cap might fit on to. Anyone know what this is or if I'm completely off the mark? I can take pictures if it is a complete mystery so far.
Author: admin