Ok the plot thickens..
I thought the problem might be the headlight switch so I pulled it out and apart then I jumpered between the red and orange wires and everything started working as it should.
So I thought it might be the switch. I went and got another switch and the horn still only works when the lights are on, but if I jumper the red to orange on the back of the plug everything works... Where does the orange wire come from? I'm assuming it's supposed to have power going to it but I cant find it in the Haynes manual..

Thanks again