Perfect opportunity for that young man to learn how to use a multimeter & learn how to read a wiring schematic!
First things first, with a vehicle as old as these are getting, start with the basics: visually & physically inspect wires & connections...assume nothing, verify that each connector is actually connected, plugs are plugged in, wires aren’t broken off or have insulation worn through which causes the wires to short out against metal components, and so on. You’d be surprised how often these things are the causes of so many problems in lighting in these old trucks.
Check ALL grounds! I usually remove the grounds, remove all rust/paint/etc. in order to ensure that the ground wires or straps are making a definite metal-to-metal connection, then reinstall & paint over. This has been the second most frequent wiring issue in my experience.
With the multimeter, set it for DC Volts (if it’s digital, set it for 50V DC or whatever is well above the max of 24-volts you’re testing), and remember that you will NEVER get a perfect 24-volt reading unless you testing across battery terminals! The more voltage drop you get the farther from the batteries, demonstrates the inherent resistance in wiring & connections...but if it’s significant, like say more than 5 volts loss at a light fixture, you may have some corrosion or frayed/broken wires somewhere in between.
With these trucks’ wiring harnesses being as old as they are (most likely 40-60 years old!), there will be wiring issues to rectify.
Do these simple tasks before you tackle more difficult ones like the 3-lever light switch.
I’ve noticed on “newer” production turn signal switches, is that the wiring inside is much smaller than the older Mil-Spec units, and the newer units will fail much quicker due to smaller-diameter wiring....I have 3 or 4 I bought NEW several years ago, and all failed the first time I tried them. I kept them thinking that I’d replace the small wires with real-size wires someday to have as spares. So if these are an issue you keep having, try to source a take-off used unit that was known to be working.
If you haven’t done Clinto’s turn signal conversion yet, DO IT!
On the wiring diagram, it’s best to go to a print shop & have it printed out as large as you can (maybe even on poster board) for easy reading. Tell that young man that the ground symbol means that the wiring is using metal components (frame, body, etc...) as large wires to return the DC voltage back to the Negative terminal on the battery...that’s why it’s imperative to ensure good ground contacts.
Good luck, report your findings back here so you can be assisted through this.
