Zed Head

Can't be sure. Not looking good. If you're getting power at the horn button wire then the connections on the way to the horn wire should be good, they use the same plugs, in and out. It might be that the light is going out because the solenoid coil is bad. The heat from grounding the circuit opens the circuit. Can you trace the green wire from the C-6 connector and see if it gets to the horns? That would be a clue.

The test light probably goes out because you're measuring the side with the resistance of the solenoid coil. When you complete the circuit and the current passes through the contacts the voltage through the solenoid drops too low to light the light. What kind of test light do you have? Another way to test the horn relay would be to connect it to one of the horn wires, at the horn location at the front of the car. Then press the button. The light should light. If it does the relay works.

It might, I'm not sure. I'd use a short Phillips head and see if could get the screws out. Or a long one, sometimes a long blade will reach through the morass and get to the screw heads. Looks like two and it should fall right out.

Should also hear the relay click if the horns are not too loud.

Only if the green wire has a good ground circuit.

Find the wires with the right colors. Green, green w/red, green w/black.

Do you have a small bumper 260Z or a big bumper 260Z? Maybe you should be using the 1975 280Z FSM. 260Z was a changeover year. Here is 1975. Maybe it has a clue. The location of the fuse box might help, for reference, and the vent pull.

Probably not the horn relay. You keep showing the picture with four black lines on it. What is that picture supposed to illustrate?

These guys have added a new feature to their graphics. Vaccination rates by county. Pretty interesting. https://covidactnow.org/?s=1971887

You shoulda said something... https://nostalgicac.com/datsun-240z-260z-280z-compressor-upgrade-kit.html

That's a popular one for the Z cars. There is info on the site about it. Here's one.

It's a rebuilt head, probably on a rebuilt short block. The N47 with flat tops would give a high CR. I don't think Nissan did that for their factory engines. Looks like somebody's home project. BRE probably stands for Big Ron's Engineering and Speed Shop.

the horn will make a horn noise.

The thing glued on with the red goop looks like a heat indicator used for rebuilt parts, to show that they've been overheated. BRE? Maybe Brock had a head ready for a race car and somebody ended up with it. https://bre2.net/the-racing/datsun/

https://bringatrailer.com/listing/1971-datsun-240z-178/?utm_source=dm&utm_medium=email&utm_campaign=2021-06-23

That is all correct. The power is used to actuate the relay, which then lets power through to the horns. The horn button always has power, that's why the horn always works, key on or off.

The diagram is not the greatest, they show power as starting at the starter, they left out the connection back to the battery positive terminal. Kind of incomplete.

Yes, the green wire is the ground wire for the relay solenoid. When you press the horn button the green wire is grounded, actuating the relay, which then passes power to the horns. You can take that green wire and touch it to a ground and the horns will get power. Honk.

eBay would be your best bet. Or you could probably cobble together the valves and a vacuum bottle. Some of the valves, and the capillary tube, are probably non-functional and/or leaky, on your collection of old parts. You might be better off to get a more modern system. Here's one. Or at least part of it. There might be threads on the site about swapping a more modern control system in. Can't remember. https://zcardepot.com/products/air-conditioning-a-c-r134a-kit-260z-280z-74-78?_pos=6&_sid=687828280&_ss=r

It will work the same with either fuse.

It's shown in the AC chapter. There are two solenoids (magnet valves) that control vacuum from a reservoir bottle that gets its vacuum from the intake manifold. This is from 1976.

It's just been burned in to my brain by the fact that my parking brake would barely slow the car down no matter how hard I pulled. It would be useless in an emergency. On the other hand though, I had a friend with a 70's Celica (in the 70's) and another friend would grab the lever and lock up the back wheels while we were cruising down the road at 40 mph. Just for fun. So mine probably could have used some work. Still, it's designed for parking.

Just posting the links to fill things out. And, it's a parking brake not an emergency brake. https://www.thezstore.com/page/TZS/PROD/24-5151-1 https://zcardepot.com/products/parking-brake-cable-emergency-rear-240z?_pos=1&_sid=9d170309b&_ss=r