Starting issues. new alternator & battery. junction box??

[Aloria]

This sounds like it could be what's happening..
Would you mind re-explaining this to me in lay(wo)man's terms?
Are the brake switch and pushrod parts under the brake pedal, inside the cab? Or are they further inside or underneath?
If it's not too much trouble and you have a photo, then I could visualize where the cushion for the brake switch is, and maybe I can troubleshoot.
Thanks!

inside the cab where the brake switch meets the pushrod(?). Anyways, over nearly 30 years, the switch hitting the rubber cushion time after time made a divot in the cushion itself. When the switch would find its way into this divot, the truck thought the brake pedal was being depressed, and kept the lights on.

[Aloria]

Thanks!
Can you describe how to locate the brake pedal switch and find out if it has failed or not?
Sounds like a likely possibility, as the brake pedal lights have been staying on (or coming on intermittently when the car is parked and ignition is off).
Can you also define what a "master" is? Do you mean a failing master switch of some kind?

And don't overlook the brake pedal switch itself, they can fail over time, when they do the fault will show as one of 2 possibilities, brake lights will stay on or they won't work at all.

In very rare circumstances, even a failing master could be the cause

[Aloria]

So, I poked my head underneath the brake pedal (good thing I'm flexible!) and I think I found the brake switch and cushion..
The cushion has a little dent in it, but not much.. however, it did seem like the brake switch wasn't being pushed all the way back up unless I lifted the brake pedal by force (which is what I've been doing with my foot).

Is it possible there's also an electrical issue connected to the switch?

It was a little gunky, so I sprayed a little lubricant on the switch and it seemed to move a bit better.
(this photo was taken while depressing the brake pedal with my hand, so I could get a clear shot of the switch/cushion)

[Burntboot]

Looks like you found it.
While the dent doesn't seem like much, its obviously enough to be causing the issue.

You need to clean everything well and verify that the switch moves freely and that the pad is not contaminated by brake fluid, causing it to soften.
-If you look further up the brake pedal arm, you will see the master cylinder push rod that attaches to the brake pedal.
Sometimes a leaky master will dribble brake fluid down the pushrod, onto the brake pedal arm and run down, softening any rubber it meets along the way and cause dirt to be attracted to the switch.

The switch is mounted in a bracket with 2 jamb nuts to allow for final adjustment, usually things set at the factory shouldn't be messed with but in this case, assuming no leaks are found and the pad isn't contaminated, I would be tempted to adjust the switch forward to take up the slack thats causing the switch to stay on.
You want to use 2 open end wrenches I think they are 12mm nuts, but could be 14mm. Hold the forward nut and crack the back nut loose, back off the rear nut, maybe a 1/4-1/2 turn, then, while holding the switch steady, finger tighten the forward nut, this will move the switch forward, once you find the right place, make sure to tighten the jamb nuts (tighten the rear nut, while holding steady the forward nut)
Don't go too far, just enough to shut the lights when the pedal hits home and please verify that the brake lights actually come on with light pressure on the pedal BEFORE going for a test drive.
BB

PS - The "master" referenced earlier would be the Brake Master Cylinder. The pedal movement is transferred to the master by the push rod that's attached to the pedal, the master is a hydraulic cylinder that forces the brake fluid into the lines to the slaves (callipers and wheel cylinders) which activate the pads and shoes.