one side of the plug is hot. i think the other side is ground. terminals on alternator are dead. no voltage between them and ground or to each other as I recall,with battery connected and ignition off.

 

so the voltage regulator has power to it when the ignition is on and it energizes the field coils when it sense low battery voltage. i found my Haynes Ford truck repair manual. not the same
as having detailed Ford electrical drawings but
it looks like direct wire from ignition switch thru a fuse in fuse/relay box , then thru instrument panel charge meter to alternator. it shows wire color. i will have to check that. a bad ignition switch should have all other accessories on as well?

 

Should be what your manual shows ..

 

To the best of your knowledge, has the windshield in your vehicle ever been replaced ?
The reason I ask is there were some really unusual problems caused by moisture leaking onto the fuse panel inside the vehicle under the driver's side dash..Those models leaked right from the factory.
If you see rust and possible corrosion around it it could create problems that at times were unexplainable. Just don't want to forget to check that out.
I'm a little curious about 3 amps draining a fully charged battery overnight to be totally dead in the morning. Anybody check the charging system to see just how much the alt is actually putting out? I know this doesn't explain the power to the alt with the key off, but might as well check evreything while in there.
Are there any add on's on the vehicle?

 

It would be one thing to have a parasitic draw, and another for the alt field to remain energized....

 

What we might be looking for is a relay that supplies only a certain component or group of components that is self energized once activated by the ign sw..
When the ign sw is turned off the relay now supplies it's own power to stay energized. Some will have a means to drain down the power and eventually open/shut off.
I have found only a few like this , but it is a possibility .

Also, sometimes a rodent will get into the wiring under a fuse panel and go to work and remove the insulation separating power and other circuits.
I agree with the logic of if something is feeding power to ign sw supplied components shouldn't other indicators also receive power. That is why I suspect it could be something the ign sw activates but no longer needs ign power when cut off. The power distribution diagram should show what other things share that power when the sw is turned on.

 

hayapower thanks for the schematic, it's a little different from the Haynes dfawing I have nut helps.
hanky, thanks I will be referring back to your answer when I get into it. there's a white plastic box under the dash to left of steering wheel I assume is where the PCM relay is.
Anodized? unpainted Metal on the emergency brake, rod and release are rusty and I have noticed moisture forms INSIDE the windshield on humid , cloudy days in cooler weather. So believe there may be a windshield leak and moisture inside the cab which you mentioned. can't remember if windshield was replaced , I have several vehicles, but its pretty clean so probably was . I will get into it deeper, trace back the alternator field coil wires and pull the PCM box and see if i find anything. don't
think bad ignition switch would power that wire without other accessories being hot also. but a stuck relay contact or stuck relay might do it. as you said.