Lost all electrical power to everything?

Where is your meter's negative probe, on the battery post or on the engine or body?

If it's on the engine, you're including the negative cable in the test. To narrow the list of suspects down, put the negative probe right on the battery's post, then double-check for 12 volts on the fuses. Unfortunately, with GM's side post design, we still can't exclude the negative cable to battery connection in the test, so for now, all we can do is assume it is okay.

The best suspect is common to all brands of vehicles today. That is a loose or corroded connection where the smaller positive battery wire bolts to the under-hood fuse box. You can often get misleading or false results when using a digital voltmeter. To prevent that, turn on the headlight switch so current is trying to flow. That will make the bad connection show up better.

This first diagram is from GM. My blue arrow is pointing to the connection to check. If you don't have 12 volts on the 125-amp fuse or any of the others, that leaves very little to check. The defect has to be in those battery cables or connections.

If you do find 12 volts on the fuses, move the meter's negative probe to the engine, check again, then move it to a paint and rust-free point on the body sheet metal and try again. If either of those comes up with near 0 volts, the bad connection is in one of those wires. Look for the copper wire strands corroded away under the insulation next to the terminals where it can't be easily seen.

Let me know what you find.

You're adding a lot of confusion by putting the meter's negative probe on the battery's positive terminal. That CAN be a valid test, but only when you understand why you come up with "goofy" readings. If we disregard polarity and any negative meter readings and just look at the values. Your meter is looking between the positive post and the fuse. Both are the same point electrically in the circuit and must have the same voltage, but we normally take voltage readings relative to something in common, which is ground or the negative post of the battery. You aren't on that common point, hence the confusion.

You had it right when you had the meter probe on ground, then the positive probe on the battery terminal, (12 volts), and then on the fuse, (0 volts). The 12 volts is missing on the fuse. Now we know all the ground circuits are okay, so it doesn't matter if the negative meter probe is on the battery post, body sheet metal, or the engine block. All three of those places are the same.

To make that even more complicated, thanks to the bad connection I think we're going to find, one meter's probe is finding ground by reading backward through the vehicle's circuitry and fuses, then back to the battery's negative post, while the other probe is on the positive post. That's where the 12 volts is coming from.

To verify if the connection is bad, follow the smaller positive battery wire down to the fuse box. This photo is from a different model. It's the stud we're interested in on your fuse box. Place one meter probe on the terminal, (blue arrow). Place the other probe on the very center of the stud, (red arrow), not on the nut or on the terminal. These two points are the same so you are supposed to find a difference of 0.00 volts there. That's when it's working properly. If you find anything other than 0.0 volts, that connection must be cleaned and tightened.

Now for the clinker. It is entirely possible right now to find 0.0 volts, but that's because no current is trying to flow. This equates to trying to find blockage in a garden hose without trying to run water through it. This is where you need to turn something on that would make current want to flow. The easiest is the head lights. When a higher current is needed, also turn on the ignition switch and the heater fan. Now recheck that voltage and I'm betting you're going to find as much as 12 volts. The only way that can be possible is if that connection is bad. Very often people will say they see tiny sparks when tugging on that wire.

Do that test, then tell me you solved this.

If possible, I'd love to see a photo of the device you found.

That connection can be tested the same way, with the two meter probes just a fraction of an inch apart, with one on each side of the bad connection. That's how we find bad connections in starter circuits, by moving from one connection to the next until we find the one with any voltage higher than 0.0 volts. But here again, and especially in the high-current starter circuit, that undesirable voltage will only show up when the circuit is trying to operate. That means using a helper to turn the ignition switch to "crank", or by using a remote starter switch.

On most models, large fuses are for the generator's output circuit. They don't involve any other circuits from working normally. Here, however, in these diagrams, you are correct that this fuse does indeed feed the Rear Fuse Box. The first one shows the Rear Fuse Box, and at the top is the 125-amp fuse from the Under-Hood Fuse Box. In the second one, I just expanded it to make it easier to see the fuse at the top.

Here's the way I interpret this part of the diagram. At the very top in the square box, it says, "B+". That goes back to the early 1900s in the first days of portable radio. Tubes needed three voltages to operate. The "A" battery ran the filaments in the tubes. The "C" battery ran other circuitry. The "B" battery provided the high voltage used in the "plate" circuits for the tubes. (You won't be tested on this later), but the "B+" designation has stuck around all this time to denote the power source, in this case, the 12 volts coming directly from the battery. Once the current passes through some other part of the circuit, other than fuses, it is no longer called "B+".

The reason this is important is right above the fuse you see, "B+ Bus" going to the left. That means it is a buss bar that all the other fuses are connected to in that square box, which is the Under-Hood Fuse Box. Most importantly, that circuit taps off before the 125-amp fuse, so nothing there is protected by that large fuse.

Below that fuse, nothing is shown as tapping off inside the Under-Hood Fuse Box or after it, other than the red wire going to the Rear Fuse Box. If the 125-amp fuse was protecting something else not related to this diagram, it would have been noted the same way as that "B+ Bus" designation did. To say that a different way, if you remove the 125-amp fuse or if it has a bad connection, everything in the Rear Fuse Box would be dead, but only the circuits coming from that fuse box. Everything coming from a fuse in the Under-Hood Fuse Box would still operate normally.

To add one more point of confusion, when the Rear Fuse Box is dead, some circuits in the Under-Hood Fuse Box will also appear to be dead, but this can be misleading. I didn't research this to verify it, but it is very common in most models to have a fuse inside the vehicle feeding one of the three or four parts of the ignition switch, including the part that turns on the low-current side of the starter relay. That relay switches on the medium-current side of the starter solenoid, (bolted onto the starter motor). That medium-current part of the system is fused in the Under-Hood Fuse Box. There's actually nothing wrong with the medium-current part of the system. It just doesn't turn on because the low-current part is not working.

This is why we often sound repetitive when we ask people for additional observations or information, such as, "do you hear a relay click", "are any other lights also not working", and things like that.

I agree, it sounds like what you found is not factory. I don't recall ever seeing a wire attached with a set screw. It sounds like that is something that was added on to allow other wires to be attached, such as perhaps for a snow plow or roof lights. Regardless, it sounds like it's wired correctly. This would be a much better alternative to the way a lot of people add wires. That is to bolt them right to the battery cable. By far the worst example, and never acceptable, is to attach a ring terminal under the cable on a GM side-post battery. Those cables make their connections where the ring of lead in the cable end contacts the ring around the threaded hole on the battery. Sticking a ring terminal in there detracts from the nice solid connection that is formed when squeezing the two soft lead surfaces together. A future intermittent connection is guaranteed to develop, and that very little additional resistance introduced means a huge deal to the starter circuit that requires very high current.

Just as bad is trying to add a wire under the head of the bolt that holds the cable to the battery. That bolt is not connected electrically to the cable. It does get 12 volts through its threads, but those threads are not designed for current flow or for excessive tightening. Added-on wires will crush over time resulting in loose connections and often some arcing that burns the lead rings away. Next comes repeated tightening, often to the point the threaded hole strips out. Once the terminals have arced away, there will always be a less than perfect connection no matter how much the cables are tightened.

With top-post batteries, we used to occasionally see where someone stuck a wire in between the post and the cable clamp. The posts are that big in diameter because that is what's needed to allow up to 300 amps to flow to get a V-8 starter going. (That does drop down a lot later, but that's a story for another day). When wires are stuffed in between the post and cable clamp, a lot of that contact area is eliminated, often the point what is there burns away from having to handle all the starter current. It's common to find you can just lift the cable off without loosening the bolt.

From what I'm visualizing, it sounds like the part that was added on is not in the high-current starter circuit and it was the best way to add on other wires. Be aware too, at least back in the '80s and '90s, GM was real good about including unused terminals in their fuse boxes for just such add-ons. Their fuse boxes down in front of the brake pedal usually had a single unused terminal marked "Batt", meaning it had 12 volts all the time, and another one marked, "ACC", meaning it had 12 volts only when the ignition switch was in "run" or "accessory".

Besides those spare terminals provided, there are a number of other way people use to add wires, and most are not acceptable for a quality job. Worst is sticking a pigtail terminal alongside the end of a glass fuse or alongside a spade-type fuse. Those used to be common when car phones first started showing up. Those terminals spread the terminals in the fuse box. Later, when the car is traded in and those accessories are removed, the fuse terminal is loose in the fuse box and causes intermittent operation. The most elusive problem is when the affected circuit appears to keep on working, but the added resistance between the terminals causes heat to be generated, and that leads to intermittent blowing of that fuse. Fuses work by their elements getting too hot and melting. The real solution is to sand the fuse box terminals clean, then squeeze them to make tighter contact. Some people used to "solve" this by installing a larger fuse, but that didn't fix anything.

So much for today's story hour. I'm very happy you solved this problem.