If the '66 is like the '67 in this regard, the horn(s) should be bolted to the driver's side fender, inside & forward of the inner fender. They are plug 'n play. If you have a base Beaumont or Chevelle 300 with a single horn, you can just plug a second one in as the wire lead is already there & taped back.
Attached is an Assembly Manual drawing for a '67 Chevelle as it was all I could find.
I hope this helps. I really find that people don't take pictures of their horn location & details, so it is hard to find a reference.
