Check for chaffing of wires around the horn bracket itself and also around the head stock. XB's are notorious for earth faults and these can show up as mild such as horn and light probs such as yours through to serious such as engine stopping completely which is what happened to mine. HD dealer could find bugger all problems.
I took mine up to

and he went through the electrics from top to toe and not only found the problem, but also took preventative measures for a few others that were waiting to happen.
Gotta love those electrics.