Check your main grounds as well, particularly at the battery and chassis. In all of my years of working on these harnesses, rarely does the main power line fail as you describe.
I pretty much agree, except to say I have not ever seen an open circuit failure, other than a fusible link that sacrificed itself due to a short to ground (as it is designed to do). Do not rule out the ignition switch itself as being the problem. These intermittent errors are often very difficult to track down, unless a person gets really lucky and happens to bump a suspect circuit just right and notices the problem can be replicated (as with an intermittent open circuit where a connection is subject to opening or closing when barely touched. The first time I can into that was with a 1973 Mercury Capri that would die at times, then start right back up, The owner was a family friend, and he took the Capri to several different Lincoln, and even Ford, dealerships. Lots of parts were replaced (ignition switch, ignition coil, points and condenser, plugs, alternator, fuel filter, carburetor (all under warranty). I got so lucky, I had a list of the many things already replaced so no need tor replicate that!
So, seriously, I drive the Capri hard (he gave it to me for as long as I needed it, and told to drive all I must), and wound up on PCH (Malibu area) one early morning, when nobody was on the road. I blasted it to 118 MPH then decided to pull into a turn-around and blast in the other direction. As I was pulling into the turn-around I noticed the tachometer dropped to Zero. The engine had died, just as reported. But, as the car was still rolling, and still in gear when I let the clutch out the engine lit right up. I pushed in the clutch to being the car to a full stop, and as soon as I pushed in the clutch pedal the engine died. I found it would only crank with no start unless I let the clutch out. As soon as I even barely toughed the clutch it would die.
I got out of the car popped the hood and looked for the clutch linkage. No linkage, it was a clutch cable, and one single wire was resting on it. It was the power wire to the ignition coil! I had left the engine running, and reached over to the wire and nudged it. The engine died. It turns out the wire had a connecter that was very close to the clutch cable, and the connector has a loosely contacting bared wire inside a rubber boot that was designed to hold the bared wire against a bolt head, where said bolt was designed to slip into the socket part of the connector on the connected wire. The repair was easy enough, I just pushed the bared wire more tightly against the bolt head it was supposed to be touching, after twisting the bared end back onto itself and twisting it into a thicker conductive end. Once back at home I made it more secure a connection, but the one little field repair would have lasted the lifetime of the car.
Years later, when working at a Ford dealership I got a Ford Fiesta in that had an engine that would die while the car was moving along a road. Remembering the Capri from years before I checked all relevant wires to make certain the same problem was not the cause. Then I drove it, a lot. I noted an engine stumble when I would hit any kind of roughness in the road. Then, while driving down a freeway, I had an instinctive thought to jostle the keys (she had a lot of keys on the key ring) on the key ring. As soon as I did that the engine died. I jostled them a bit more and the engine came back on (manual tranny, at freeway speed). I was able to replicate the problem at will. The problem was an internal contact inside the ignition switch that opened with the weight of the keys hitting at just the wrong spot. I replaced the ignition switch, test drive again to make certain the keys being jostled would no longer cause the same problem.
Anyway, the 2nd story is why I suggest you do not overlook the ignition switch. As I recall there are no connectors between the Yellow Circuit 37 wire and the ignition switch, except at the ignition switch itself, and at the starter relay. Although I would not totally rule out a broken wire inside the insulation, I also thank that would be a very long shot, as there is nothing causing the Yellow wire for Circuit 37 to flex back and forth anywhere, much less enough to cause flex fatigue fracturing inside the yellow insulation sheath. It could be a slightly loosened connection at the starter relay where Circuit 37 begins. but unless someone did some work in that area and did not tighten the retaining but at the large positive cable terminal on said relay it is not likely that came loose.
Being this affects the headlights and ignition tells me it is not likely a problem inside the ignition switch. Steve McMahan shared an important experience. I do not recall seeing the junction block he is referring to, but I have no reason to believe he is wrong - especially considering his correction fixed his problem. But, I also feel the ground circuits need to be checked. If your schematic does not show where the various ground connections are located, the Forel Publications color wiring schematic for 1973 Mustangs does. It not only identifies where the grounds are located, it also shows where all connectors are located, and what the connectors look like. It is, in my opinion, the first really good schematic for the First Generation Mustangs, as the prior schematics do not go into the kind of detail the 73 manual does. If you are interested in the Forel color schematic you can acquire it for a very low priced at the following link:
https://www.forelpublishing.com/clickbank/index.html#1973
If you ever find you need a good engine vacuum system schematic their 73 Shop Manual shows nearly all engine vacuum calibrations in Volume 6 of their Shop Manual. There are two 351 4v vacuum schematics I found, and added to my Shop Manual Volume 6 file (after removing their silly copy protection on the file). Their shop manual PDF is also highly recommended, Attached are the 2 missing 351 4v vacuum calibrations in case you need them.
