Join Date: Feb 2013
Location: Aussie in Sweden
Sounds like a ground/earthing issue.
Usually, at least in the case of my Porsche 944, one of the wires for the horn unit is attached directly to the metal of the chassis. In my case with my 944, it was corroded and needed to be cleaned out in order for the horn to work again properly.
This is just a wild guess but to me it sounds like the grounding/earthing of your horn where it is attached to the body/chassis has been corroded and is causing this. You should be able to fix it simply by cleaning the area of any grime/muck/rust or worst case, detach it and move it to another clean part of the body/chassis.
2000 Alfa 156 Sportwagon 2.0 TS CF3
2002 Alfa GTV 2.0 TS CF3
1983 Porsche 944