Kickout, I don't have a schematics set for this game, but I've worked on several of these, and they are always a problem, so to see that someone else has worked on them prior to you shouldn't be a surprise.
If I recall, there are 3 of these in the game. Their sole purpose is to shut the GI off at certain times of the game. It actually cycles them on and off so it appears they are "flashing". One of these is in the backbox on the back of the light board. The other two are right near each other in the bottom corner of the playfield if I remember correctly.
Being that they are just a simple relay system, there is a low voltage circuit that turns the relay on and off, and then the GI is wired through a set of higher voltage contacts and the relay acts like a switch, (normally ON of course) and then off when the relay is actuated. So the usual GI problems that exist on the header pins at the boards also happens here at the connectors and pins. Typically they get burnt, oxidize and burn even further. I've seen some so bad they desoldered themselves from the boards!
In any event, you should be able to trace the GI wiring at these relays and understand that they are just a switch in the middle of the GI wiring... The only other wires are the lower voltage wiring that actuates the relay in the first place.
I know that may not be the help you were looking for in getting a picture of the wiring, but understanding what should be there, may help you figure out where that "extra" black wire should go. Check a set of schematics if you have them to be sure.
Good luck.