79 911 SC Fuse Box System Failure
 

I'm having an electrical problem with my '79 911 SC. This problem was initially discovered when I found a wire getting really hot, and my horn intermittently honking.

History!
I updated my fuse box ~6 years ago to change to ATO fuses from the old fuses. I did not change the layout of the fuses, but they weren't stock to start with and they did have a headlight relay kit already installed.
My update went without any issues and everything worked great for years.
About 3 years ago I added a relay to the running lights, so they could only turn on when the key was turned to Accessory, On, or Start. Again, it didn't have any issues.
They wire overheat may be related to a stalk issue I'm having, where often the stalk contacts for high beams and low beams are on simultaneously.

Where I started last week.
My horn started honking while driving, so I pulled over and the fuse box itself was warm to the touch. I checked at home and a particular wire was fairly hot, and had significant heat damage. Also, my fuel pump relay which was next door to it has concerning curves on the sides. But the car still ran.
My fuse box rebuild was done with ATO fuse blocks, so there was no fuse bus between any of the tops of fuses. The stock fuse blocks have these at various spots, so I had replicated those buses with short lengths of 10 gauge twisted copper wire. As a result, there were several "hops" to bus together different tops of fuses. The wire that was frying was a hop on the Accessory-Controlled bus. Specifically, this hop joined the positive voltage over to a fuse for the cigarette lighter plug (nothing plugged in) and something unknown. From there, there was another bus hop to get to the high and low beams and the running lights. So this wire had a LOT of current when both beams and lights and all were running.

What I did first to fix it.
This past week I resolved to fix the giant mass of hops leading the current fuse-to-fuse. I soldered 3 wires onto the incoming Accessory-Switched voltage. I sent one of those off to the high/low/running light section of the fuse box. I sent one to the cigarette lighter, and I sent another to a few other fuses which hadn't been in the path of the fried wire, but were part of the original bus. I got everything all tinned and heat-shrinked and installed snugly. But I discovered that the entire Accessory-Switched bus was misbehaving and sometimes shutting down entirely, sometimes taking other parts of the system with it. However after a few minutes, it seemed to recover (only to fail again when I did more things).
http://forums.pelicanparts.com/uploa...1627771080.png
http://forums.pelicanparts.com/uploa...1627771080.png

The play-by-play video.
This video (https://youtu.be/__dZ-_F0Ges 10 minutes) shows my work trying to reproduce the system failure (audio gets way out of sync, will try to fix it). I have pulled out the fuses on the Accessory-switched bus so nothing should be drawing power there. Then I show I'm getting 12 volts at Always-Hot, and 12 at Accessory-switched when I turn the switch.
I re-insert some fuses and see the running lights come online. The low beams work once I have their fuse in. I put in the high beam fuse and turn them on, and everything goes down.
The Accessory-switched power is down at 2 volts, but the always-on is at 12 volts. This state persists after I remove all the fuses and relays.

Current state.
Regardless of fuse or relay presence on the accessory bus, I'm getting 12 volts at the always-on bus and 2 at Accessory-switched bus. I'm also seeing the speedometer bump up to 20 or 30 MPH when I turn on Accessory, but it goes to 0 when I turn on the lights. It also goes to 0 if I go to the On key position. Video of this (https://youtu.be/VWasYeKgtog 3 min).
Right now the system has "recovered" back to normal, although I have all the fuses on the Accessories-Switched bus removed, and all the high/low/running light fuses removed.

I'm not sure where to check next. I tested all the fuses and replaced all the relays. Maybe something somewhere in the harness fried and shorted? I think I'm about to learn a whole lot more than I want to.