Pictures of wiring please

That wire on the bottom right of the starter relay is a short waiting to happen. Based on those pics, I'd say that the whole car needs to be looked over from an electrical standpoint.

I agree that the wire with the stepped connector looks like the one that goes to the starter relay.

Del has posted exactly what you need as far as identifying wires coming out of the bulkhead connector. Depending on options, you may not have everything listed in the diagram but you should be able to figure out what you are looking at. That listing is not in those diagrams I posted so YRMV.

Again, that wiring needs some attention before stuff starts burning up. Corrosion is usually the main cause of high resistance. The nut on the stud on the starter relay is pretty crusty looking and there is a lot of current going through it. If you get a higher resistance value on a circuit that isn't designed for it, the potential for melting stuff is greatly increased. Plus, old wiring gets brittle with age. The insulation cracks and that's when things start going bad. Not to mention people rigging stuff over the years.

Case in point; I was at Carlisle last weekend. These guys were putting the original '68 Don Grotheer Road Runner Super Stocker around the grounds when smoke started coming out of the hood. I happened to be standing right next to it when it happened. They opened the hood to see what the problem was. They were going crazy looking around for what was on fire and luckily I saw it immediately and pointed it out. The ballast resistor wires were completely melted and smoking badly right behind the air cleaner. This is an original car with the original wiring. Good thing they caught it or that could have been ugly.