I had to strip mine down recently, loads of corrosion on the contact surfaces and the return spring was broken, it's all straightforward to fix though, once you have separated the two halves of the casing and taken the plastic horn button off (held by a small screw on the underside of the button), you can lift the switch out, it has a thin (tuffnel?) cover, gently pry it off then you can take out the moving part of the horn switch and clean up the contact faces, I replaced the broken spring with one from a ballpoint pen, then you can glue the switch cover back on with araldite or similar, then reassemble the cases and job done, nothing in there to worry about.
