Does anyone have a lead on a set of 12v 544 horns, or the lover tone one. eBay is no help and the Manuals has next to nothing about the inner workings of the horn unit so I'm just poking at it with a stick not understanding how it really works.
Typical construction of electromechanical horns is based on a coil and armature with an electrical opening contact attached, such that when armature is powered and makes an excursion, it opens the series contact which breaks the current and causes the armature to return to its resting position...this cycle repeats for as long as power is applied. A diaphragm connected to the armature moves air and thus creates (lots of!) sound. The contact is often adjustable...look for an adjustment screw...sometimes adjusting this can bring back a numb horn. Failure modes are often caused by internal corrosion making it difficult for the armature to move normally, or carbonization of the opening contact, causing it not to pass current...checking this will clearly require going inside...but often, an internal cleanup/lube will bring them back to life...this might require grinding away or drilling out rivets holding the assembly together. Use earplugs when testing to avoid surprises and possibly soiling yourself!