the horn (2 horns!!) have one Relais ref the drawing above. The issue is normally with the horns as very old so called WAGNASCHER HAMMER, where a big electro magnet (stack of ferro metal) driven by a coil as part of this magnet. The ends of the magnet face towards a metal plate close to it. When the horn button is push current enables the magnet to work, hence pulling the metall plate inwards. There is a contact (kind of relais contact type) fixed to that plate which as soon as the metall plate got moved towards the magnet the contact opens, hence the metall plate moves back in place closing the contact, and the process repeates.
The horns suck under normal conditions a LOT OF CURRENT, guess each 5..10 AMPS..
So go test them on the bench - with ear pluggs 
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The metal plate could be rottet so does not bend, so full current for ever..vs intermitting.
Test the relais, as you now know where it is located, the switch is normally not the issue as low current only for the relais, say 0,5 Amps. If the horn makes sound it is NOT the switch nor the relais..
Matthias