This is a long shot, but maybe someone here has a set of freeway horns for 1971 3.5 or 6.3. These are the optional set which mount below the front bumper- they have little square chrome or stainless grilles on them. (they were the 'country' part of the 'town and country' two-tone horn package, which was an option). They were offered on 3.5s and 6.3s.

My 3.5 has a set but they don't sound like they ought to, and we can't get them to- I think they are damaged inside by age and weather. If anyone has a set of these, I would be happy to buy them, if they work and are good and LOUD. PM me please if you would. Thanks for reading this.