76 C20 454 harness mess

[TheC5Surgeon]

Just got the truck home and started to dig into the hectic wiring situation. My electrical knowledge is admittedly pretty weak so I am learning as I go.
So a little background on the truck. At some point there was a second battery installed (unless some models came with two) along with subwoofers, amps, aftermarket radio/speakers, CV radio, and some other crap that I am in the process of tearing out. There are cut wires, damaged terminals, and degrading connections all over the truck. When looking at the fuse panel underneath the dash wires are shoved in behind many of the bulb fuses and there is a huge mess of wires tied around each other going to and from all the accessories.
There is a constant drain on the system because the batter would be dead after a day or so if left connected. Now I cannot get enough juice to start the truck even when leaving it connected to a trickle charger over night. The batteries are old and need to be replaced but I would like to figure some of the harness issues out so I can enjoy the truck while saving for the diesel swap.

My first question is mainly where to start on something like this? I have downloaded the 1976 wiring diagrams but they are pretty tough to follow. Any advice would be helpful.

I attached some pics of the wires going from the drivers side battery (the aftermarket one I believe). I can take plenty more of the engine bay and inside the truck as well if needed.

 

[Big Chip]

https://www.gmsquarebody.com/threads/1973-1991-wiring-manuals.14498/

 

[75gmck25]

The 2nd battery was a fairly common factory option for towing or a slide-in camper. On my ‘75 there is a battery tray and an ignition activated relay that connects the 2nd battery to the charging circuit when the truck is running. When the truck ignition is off the battery is isolated and it has a power junction so you could run power to a camper. However, I think the factory install ended at the power junction and all other power wiring was dealer installed.
My truck also has a factory T in the wiring running to the rear lights and it’s up near the engine compartment. The 2nd set of wires was for camper lights or trailer lights.

 

[TheC5Surgeon]

Trying to simplify things with the harness. One thing i'm confused about is in the pics. That relay went to the accessory battery but why does that brown wire (which goes through the firewall and attaches to the pedal sensor along with the orange wire and then to the fuse panel) also connect there? The orange wire then goes to the transmission as a part of the kick down. According to the wiring diagrams there is a brown wire going to the distributor but this is a striped brown/white and also going through the firewall (the diagrams do not show this). I am confused to say the least. Trying to remove as much of the accessories as possible to simplify this thing. Any ideas on what this is and how I should remove it? Thanks

Matthew