Odd one.
With the column connector disconnected, you should not have a problem with the horn contact going to ground.
With the brake light switch disconnected (you did pull that 2-wire plug, right? You should not get any feed to that circuit.
However, there is always the possibility of the plastic/phenolic-resin connector housing thingies being so old and carbon traced, or a crack has formed between two contacts, that a little bleed-over is being allowed.
Less likely in the type of plastic they use in the column connector, but hey, what the heck. Pull the horn wire out of the connector to see if it's shorted to one of the other wires somehow.
Yeah, I know, those are such long-shots as to be too crazy to even contemplate.
But so is any kind of a short to air, which is virtually what you're finding at this point.
Which is why you might want to give it a try.
For precedence, I've experienced two occasions when the plastic of an electrical component became a conductor. One was the fiber body of a points set on a motorcycle. I forget what the second one was, but remember it baffling me for a time.
Try a known-good headlight switch too, if you can beg, borrow or steal one.
Maybe it's still in there somewhere. Maybe not, but it'd be nice to know for sure.
And while you're at all this, run a fresh ground wire from the dash to the body too.
You never know where those pesky electrons are going to try to run when things become sort of un-grounded.
Good luck. Good detective work btw. Lots of work, but you're sure narrowing it down.
Paul
