Wov.. I once had a Mercedes Horn .But there was nothing strange with that . + and - and switch button Pretty certain I remember right.
tututututut It was Autobahn Loud..
Gave the horn to a Friend that within the Month had scrapped the car with the horn still in it . That is for helping.
I Reckon this is a more modern thing please give more info.Type .Sitting on what car. Specs and or pictures
Bosch is good stuff usually .. I don't know of any better brand.
Suprising if it needs a relay but there are of Course different ones And can be from a long Haul truck even