It's simple enough to get into to extend the wiring from the orginal horn, but a pain getting the plastic off to get access. From what other people have told me, there's enough room in there to fit some serious horns, but there may be a need to fit a relay, to prevent the wires getting fried.
Got to admit, they do look a bit sad when you start stripping the plastic off.
Pan miles so far... Loads.