40 open horn issues

OR,, the horn is bad and pulling B+ down. Put a light on the leads with horn disconnected and watch if B+ stays up or drops. If it drops, start looking for a bad crimp.
You can always jump the switch with a screw driver to check there.

With the horn still connected or a light in it's place?
An electric horn is just a voice coil that makes a this disk vibrate.
If the disk does not vibrate (bugs, salt or rust), no sound but still a coil load that will still drop B+.

Ya didn't mention that..
Then carry on;
Ya got a bad wire or crimp somewhere.

Do you have a horn button on the tower and helm?
Same poor results?

Like you tried to say in post #4, Bad B- (grown).
Run a fresh negative lead if you can,, from a known good negative bar.

IMO, shrink crimps should be SOP.
For those in RosaLinda and Brother Carl (who hates my acronyms) , that is Standard Operating Procedure..
Just ensure you crimp onto fresh & bright wires or all is counter productive. This includes replacing any black wire runs.

That is what I'm typing about..

Sadly, even when using marine shrink crimp connects, nothing is water proof. In our better dreams weather resistant may come to play well.
With some grease, were just slowing the weather intrusion into the electrical connection down a little.
Mother Nature always wins. We can just slow down her fight sometimes.

Been fighting with that ***** for very many years now. She has fangs....