definitely sounds like dodgy wiring to me, when i met my boyfirend i had to rewire his corsa because every time he turned the neon lights on under his car the window wipers went on full blast!
if you can't see where its going wrong it might be worth following the wires from the horn back to see what circuit its wired into (im assuming the same one as your fog lights) , failing that there are always spare fuses in your fuse box and you can rewire your horn through a different circuit.
a lot of car companies wire them into the same circuit because they're called 'occasional usage' circuits (less than once a week usage)
so its most cost effective for them to join them together.
hope this helps x