How can the Engine keep running when the ignition switch is turned off or the harness is unplugged under the dash?

How many people have had trouble shutting your truck off INSTEAD of not being able
to get it to start?
I have a 77 W150 with 400 and upgraded to 100 amp alternator a few years back.
I bypassed the amp gauge in the dash a couple years ago ran new power wires in the
harness under the hood & dash. solid wire through fire wall to avoid bulk head
connector issues.
Have had no trouble until just the other day when I turned the truck off and it wouldn't
stop running. The power to the cab seems to turn off like the radio for instance. I even
unplugged the ignition harness under the steering column. The only way I found to shut
it off is to unplug the battery and un-wire the alternator charging lead. The other option
would be to pull the red wires/plug to the ballast resistor. Re-connect wires and it will
start back up with no trouble.
I have the wiring schematic, but I'm having trouble confirming what the key switch is
supposed to control in order to shut the engine off? Short of tracing wires through the
harness. I know something has got to control power to the coil or at least thats my
guess as to how it is supposed to actually shut off?
I know the Voltage Regulator and ICU are famous for going out without notice! It will
pull down the battery within a couple days if left connected.
I would like to trouble shoot and understand what happened instead of throwing parts
at it. BESIDES I'VE NEVER HAD TROUBLE TURNING A TRUCK OFF BEFORE!!!
ALMOST ALWAYS HAVE TROUBLE STARTING ONE IF ANYTHING!!
Very curious what happened and why? Something has had to fail shut that would
normally be a fail open contact in my mind?

Thanks in advance for your comments!