Tfain, I know your frustrated and you want this to work like yesterday but you are going to have to listen to people on here.
I sent you over some diagrams earlier in the thread and it clearly shows what that wire is on the diagrams and here is a hint, it is a ground. I pulled it up and found it in 5 minutes.
Again, I'm not being a hard *** or mad, I've been observing this thread.
Wiring Gnomes are always fun to deal with but you got to establish the factory wiring from the newly introduced rats nest. Start at the tail lamps and work backwards. Get the manual out or diagrams and start following the factory color coded wires and fix the splicing, patch up exposed wire sheathing, clean/fix grounding points or make news wire looms.
Earlier in the thread I mentioned the PO had used lamp wire on my pile. He cut the actual factory wiring and ran a wire to the brake light switch and one to the battery, with lamp wire. It was amazing, though it did not for some reason.
I wound up making my own harness and looms from the tail lights up to the connecting harness block, brake light switch and headlight switch. The wiring was so far gone that it looked like a shotgun got a hold of it. Trust me, I know your pain and frustration.
Take some breaths and start basic. Get a testing light and multimeter, these tools are highly valuable, I cannot stress that enough, along with what has been stated by others. Oh and wiring schematics!!!!!!!