I ran into something like this when I bought my '85. The PO hadn't really done anything good or bad (or anything at all except own it for 4 months) to the truck, but the guy that owned it before him was a real "Harry Half-ass" when it came to wiring.
The wiring under the hood was functional, but haphazardly arranged. Oil pressure sending unit wire pinched under the head of a bellhousing bolt, things like that.
Anyway, to the stereo wiring. There had been a generic DIN head unit in the thing when the PO bought it, but he said that the stereo wiring was so FUBARed that he gave up and just used a boombox (he barely drove it anyway). I got under the dash and FUBARed was an understatement. Those inexpensive plug adapters you can buy, so that you don't have to cut the factory wiring, were unknown to the guy that messed all this up, so he cut the factory plugs off and put various colored wires on the freshly cut leads.
I really hate half-assed projects that would have been better if they'd never been started. If they couldn't afford to to it right, they should have just left it alone (like the last owner wisely did) until they had the money.
I'm not even going to mess with that electrical disaster. I'm replacing the cracked dash pad anyway, so I'll just put in a new pair of 3-1/2 speakers and run new wiring from those to the head unit area at that time, as well as new power and ground leads. I'll find the ignition and illumination leads, and use the ignition lead to just pick a power relay for the stereo hot lead.
I'll be running a thicker hot lead to the powered sub anyway; nothing special, probably a Kenwood KSC-SW1 for it's nice, small size. I'll worry about doing something with rear speakers later.
