the schematics seem to show that early horns didn't rely on a relay, therefore the rubber mount is not for electrical isolation, it must be purely mechanical isolation then?
early horns have two connections, one is battery hot from the back of the amp-meter and the other goes to the steering wheel switch for ground.
horn current does not register on the battery charge/discharge indication, the horn power is tapped off on the battery side of the amp gauge.
however all my schematics are for USA model vehicles, and the Canadian vehicles i deal with have a different horn setup than the common south of the border ones people discuss on here. and my earliest trucks all have two horns and a relay and the horns have only one wire connection and MUST use the chassis ground which would not be compatible with a steering ground switch unless you use a relay (which is not on any schematic i found)
so you could be right about needing a relay, if you have a horn that only has 1 wire connection, but then that means the rubber mount must have a feature to connect the chassis ground to the horn body around the rubber vibration isolators.
we probably need a time machine to know for sure, unless someone has very good original "as built" documentation.
-s
