This truck had a flat bed on it when I bought it with no way to separate the harness from the bed. I sold it and just chopped the wires so the guy who bought the bed had hopefully an easier time wiring everything in. I put a regular box on it now, but I’m still trying to track down the wire bundle from the block under the hood to the connector at the bed.
Just buy a new harness from EZ or Rebel wiring. Less than $300, gets you a new fusebox (cigarette pack sized), all new plugs, deletes the fusible links, every wire is fully labeled, no corrosion under the insulation… A couple pack of plain fittings, some heat shrink tubing…
Buy once, cry once, and only have to wire it once.