[steve240Z73]

This can be a little complicated if you can't read the 240Z wiring diagrams.
From the battery, the main power cable goes from the positive terminal to the starter. From the starter, there is a fusible link that connects the positive to a white wire.
The white wire goes to the fuse box for the brake and horn circuits. The white wire also goes up to the ammeter gauge.
The alternator positive is connected to a white/red wire. That wire branches to the fuse box where it powers the parking light, dome light, and cigarette lighter circuits. There are two branches with inline fuses going to the accessory relay and defroster relay. Other branches go to the ignition switch and headlight switch. The most important branch goes up to the ammeter gauge.

So going back to your picture:

The five fuses I circled below the headlight fuse should have voltage to ground at all times.

 

[steve240Z73]

Here are cleaner scans of the wiring diagrams. I made them because I didn't want to have to find my FSM every time I wanted to look at them.

 

[steve240Z73]

The best way to repair it would be to put a 7.8 mm connector on it and get a new fusible link. You can find them here: https://www.zzxdatsun.com/catWiring.php
He only takes checks, and you have to pay in advance, but Mike has been involved with restoring Z cars for a LONG time.

If you want to use a credit card and pay more, you can find the link at Motorsport Auto: Motorsport Auto - The Z Store - Fusible Link, 71-73 240Z
The connector is available from Vintage Connections: 7.8mm 1-Pin Latching
You may want to buy two or three of those connectors if you're not experienced with crimping wires.

You will need an appropriate crimping tool. This one can do 10AWG wire: https://www.amazon.com/gp/product/B0978S514B That will work with the white wire in the wiring harness.

When you get the parts, I can give more instructions on how to fix the wiring. It wouldn't hurt if you could post some clearer photos of the wire hack, too. Make sure the photos focus on the wire.

 

[steve240Z73]

Can you take photos of where the wires have been cut? And if you want to use wire that matches the color scheme of the cut wires, you can probably find it at Vintage Connections.

The white wire in the engine bay is approximately 10AWG. Keep in mind that in the wiring harness there is also a 14AWG wire that runs from the connector for the voltage regulator to that larger wire. They are spliced together in the wiring harness.

 

[steve240Z73]

Would you suggest I buy a new harness? For some reason I can't add pictures now, but the fusible link it soldered wire to wire and covered in heat shrink (in yellow -- pic above) with the white/blue wire

You could maybe find a used wiring harness, but it HAS to be for a 73 and for the same transmission (manual or automatic) as the dash harness. It has different connectors from the other S30s.

 

[steve240Z73]

Hey Steve, hope all is well - what brand wire would you recommend for the 10 AWG, I got some from Amazon but hesitant to use it due to the reviews of overheating issues

I haven't worried too much about brand. Make sure you get copper and not aluminum wire.
This place has GXL wire. It is rated up to 125°C (257°F). 10 AWG
I have not personally done business with them.