There's not too much to the engine wiring on these trucks.
GM used Crosslink Polyethylene wire jacket in these trucks so the wires are usually in reasonable shape.
There isn't an engine harness per-se... more like several individual wires. You have the following on the engine.
- Oil and Temp sensors for the gauges or lamps. Two wires.
- HEI Tach and Power. If you have tach it's two wires. If not it's one.
- Alternator... charge, sense, and excite wires.
- Starter... Crank, J-Stud feed, Bulkhead power feed, and sense wire to the alternator. And the fat electron hose from the battery.
- The single AC compressor power wire that runs from the Low Pressure Cutout switch and wyes to the idle kicker solenoid.
- Ground wire from the battery on the alternator bracket.
The forward lamp and horn wiring is the closest thing to a harness in the engine bay. It's six wires plus some grounds to the rad support. If this is damaged you can pull it out and lay it on a sheet of plywood and lay out replacements for any damaged wires.
The wiper motor and heater/ac are technically separate individual harnesses. Not too many wires in either. You can unplug them. lay them out on the bench and install replacement wires with new terminals if needed.
I often see butchered rear lighting wiring harnesses if you want to call them that. This is the tail lamp wires to the rear of the frame. For some reason trailer installations meant popping on Scotchloks or chopping off the wires and adding open butt connectors that turned the wires into a green corroded mess. It's four 16ga wires with Packard 56 terminals on the bulkhead plug end and Weatherpak terminals on the tail end... LH Turn/Brake, RH Turn/Brake, Backup, Parking. I usually add un-terminated Blue and Orange 12ga wires with 16" or so of slack on the forward and rear ends for trailer brake and trailer breakaway battery charging. You re-use the bulkhead plug end and replace the pre-1983 molded plugs with the 1983 and later Weatherpak.
The cab harness is more of a real wiring harness... they're fairly specific to the options in each truck. GM factory staff must've had a real picnic keeping track of these on the line. I generally repair these so I don't loose functionality.
Even inside the cab there are bundled sections of wiring that are separate. Like the NL2 dual tank switch wiring, power windows, power locks, the dome light, and cargo light.