Still no updates on the mystery switch huh?
Sounds like the consensus here is auxiliary lighting which totally makes sense, as the rocker is right by the light switch.
When I saw the pic of the blue wire with the white strip I had another thought. Years and years (and years and years and years) ago, I installed stereos for a living. All mobile audio in cars, trucks, boats, even a couple of airplanes. Anyway this looks a lot like how some shops would add a remote switch to bypass the accessory ignition switch to get power to the audio system without having to put in the key and turn the car on. The idea was that you could run your system w/o the key in the ignition so no lights or annoying door bell if your doors were open. I’d also see this hooked to amplifiers to manually disconnect power so as not to drain the battery. That was more common when adding amps to a stock stereo system. TobyU brought that up earlier too. Amps may have been pulled when the truck was sold and they just left the switch there.
Is the stereo in your Expy aftermarket? Whether it is or not, try turning on the stereo w/o turning the ignition to the accessory or on positions. If nothing, hit the mystery switch and try it again. Just a thought.
If that’s not it, then I really hope it’s for a trunk monkey!