My understanding is that you need a relay in order to keep from frying your switch. Somewhere here on the forum is a write-up where someone (MrClubike, maybe?) fashioned one.
I was going to ask about this again soon since I want to change my single to a dual.
I think Cycleterminal stopped making them. Eastern Beaver might still do them but they could be pricey.
I have wire and connectors, just need to get a relay and figure out where everything connects.
Hopefully one if the electrical/horn experts can offer some advice?