Thank you so much for all of this. My 1946 Plymouth's horn is now working!
To add a wrinkle, I needed to polish the brass 'T' below the tribar (polished everything just in case). To troubleshoot, I set a multimeter to continuity mode and connected to the wire exiting the steering column and ground. After disassembling the horn, I got continuity from the metal components but not after putting the horn ring back on. After polishing all metal components, it worked perfectly! The horn relay was also bad (confirmed by touching two terminals of a 9v battery to it without hearing a 'click'). An HR-114 from Amazon did the trick.
