I've tried this a few times with old car horns..
Go to the pharmacy and get a bottle of 91% rubbing alchahol and just pour it on in
to the horn (which should be disconnected at this point). fill it about half way if it
holds fluid, and shake the hell out of it. Sometimes it's enough to wash off any
loose dust and grime without having to take it apart. Use the 91% because the
regular stuff has more water disolved into it, and putting water on metal or electrical
circuits is generally a bad idea.