I'm far from a pro but have someone sit and attempt to blow the horn while
you (like a fool) lay on your back under the front right bumper and tap the
horn several times with a screwdriver handle or something like that. 
After awhile you may here something that sounds like a mouse hiccuping. 
That's the horn.  As you work at it it will get louder and louder and then
you won't look like a fool any more!   Worked for me.  Good luck

Wendy
'85 Westy,   "S-CAPE"