What caused this electrical carnage?

[u2slow]

Imho, those burnt wires are from corroded crimps and contacts; (higher resistance) and/or possibly the GP's running longer that intended.

Each GP pulls around 15amps so the wiring needs to support it. The factory is a little chintzy on the wire size because they're not meant to energized for very long.

 

[HammerDown]

Update... until the Amazon power/disconnect arrives, I isolated the big yellow wires and also put new eyelets on the burnt wires at the fender mounted relay.
Cleaned up all connections, used heat-shrink, kept my fingers crossed and the old girl started like the day I bought her new in 1988.
After seeing the connections for those wires, I would believe regular 10gauge male/femal bullet connections would also work.