The original harness probably has seen it's day and needs to be refreshed/replaced.

On the 70's I think you take a hex headed screw off of the corner of the fuse block on the fuse side and then push the clips in to slide the fuse block off of the bulkhead connector. Some years had the screw others did not.

The fuse block will reluctantly pull away from the bulkhead harness to allow you to inspect the wires behind it. If you can see the wires going into the ammeter that is another good sign of the condition of the harness.

The Large Black and Red wires are from the engine compartment that go to the ammeter and power the harness. They frequently burn and cause power shorts where stuff that shouldn't get voltage gets voltage. I had one where the park/side marker lights would not turn off but I could dim them with the dash dimmer. Funny stuff sometimes.

If you need to rebuild it, it is pretty easy to do.

Pulling the dash makes it easier but I have removed the harness with the dash in. I have even rebuilt the harness in the car. Basically you carefully unwrap the harness and follow the main power wires until you see what they melted to. Fix that wire and replace/repair the main wires. I prefer to replace them with slightly larger gauge wire and run the new wires to pass through terminals rather than back into the bulkhead connector.

Let me know how the wires look.