I hate dealing with hacked up wiring. It wouldn't take much of that before I'd decide to rewire, but that's me...
A pickup diagram if you can find one should be very close to exactly what you need. It won't have things like electric shift for a 2-speed rear or the speedometer box that goes with that. Its possible you have a gauge or two that won't be represented, and of course there are lights that may not be on any diagram.
And any hacked up harness has some things added to it that may be incomplete and won't make any sense.
I once bought a truck and immediately upon getting it home spent a couple hours with my wire cutters just removing loose wires that didn't go anywhere any more.
