Thanks for the reply and diagram Dutch, am on a trip in Laos just now, so my internet coverage is sparse.
When I installed the horn I used the relay and connected a wire directly to the battery with a 25amp fuse, what I think is, when you use the horn you only get power to the horn for 1-2 seconds to stop you continuosly using it, as a BMW idiosynchrasity, is yours like that?