Darrell, I don't know what stage or condition the rest of the truck is in. My suggestion is for a truck needing refreshed/rebuilt/restored. What I do is similar to what some have said. The members above have put you on exactly the right path with great info I agree with 100%.
Do the harness AFTER you Take out and take off, all that stuff you said does not work. Actually take out/takeoff every electrical component. Lights, switches, motors, gauges, etc. Take each one to the bench with a 12V power supply. I use a battery charger. I have fun "rebuilding" each electrical component. Testing operation, sanding the contacts, soldering/crimping where needed, cleaning, shinning.........figuring out what is wrong with each and getting it working or replaced.
It's rewarding and very good experience.
You will find that most of the components are all the "same". I even take switches apart by bending the tabs that hold the phenolic back or plate, cleaning the contacts on rotary, toggle, push-pull or any sort of switch. Switches are elecrto-mechanical. I too was an engineer but these components are not difficult to understand, quite simple but well done. Motors are all the same, blower, starter, generator. Lights are all the same, socket shell, spring loaded contacts, bayonet bulb. You need to shine up all the copper and brass contacts/terminals and bits inside and outside the electrical components. Repair any broken mechanical or electrical parts they have. Test continuity and operate them, as said, with 12V supply. You may want to rig an inline fuse on the positive lead of the power supply, with a 5 amp fuse just to be safe during testing until you know what is wrong with the component. You can use a 12V battery if you wish if you don't have a charger.
All these components are grounded via a super long and multi piece return path back to the battery. Mostly accomplished by intimate metal contact and a few wires/braided/cables. A faulty ground path is usually the cause of electrical problems more than 50% of the time. So that is your other task before the harness install. Inspect, clean and be sure there are all the grounds. Battery to frame, frame to motor, frame to body. In any order you want. All those items are mounted with rubber so a flexible ground wire/cable/braid is necessary, Bolted to a super clean bare metal spot. Do all these things first. The harness install is pretty easy and will make a lot of sense if you just look at each wire on the schematic and see how it supplies fused/switched/unswitched power to each component. That's all most of these wire do.
Any more info on this pass will be boring, you will have questions as you go. We are happy to answer. After you do the above, the process in general is to mount the fuse panel and then route the input wires from the battery to the fuses (or direct to a component in a few places) and route the output wires from/thru the fuses to each switch or component or light or motor or gauge. Then there are some ignition and charging circuits. Not a lot of wires really. Not over your head. Don't throw away ANY old connectors, clamps, contacts, switches, terminals, braids or funky looking electrical bits.
