Transistors are my friends - I used them to turn on my LEDs under my doors!
I have to think about it more, but I think you can use a PNP transistor from radioshack to do this. You will need to hook one end to ground, one to the source that gets the 2 volt turn on signal, and one end to your horn which will be powered. I'm not totally positive on this, but you can test it they only cost a few dollars at radioshack.
I am NOT an electrical engineer. I figured this out with help from a member on here and trial and error. I spent probably 100 hours or so doing lights on my evo and 04 edge.