Well, it's a shame you could'nt resurrect it from the dead by taking it apart and cleaning up everything...Sometimes the diaphram and/or components just give it up...Don't be in a big hurry to find one and you might run across one that needs some cosmetic work they pop up on here from time to time at a reasonable price...
The 73-74 road runners had a single horn located on the passenger side fender area where the factory hood pins are located {in front of the rad support}...Look closely and you'll see the round horn bolt mounting hole with a small indexing hole close by where the horn's tab fits...The original horn attaching bolt had a captured,serrated washer and may have left teeth marks where it was seated on the fender...
And on that note...
...Sorry could'nt resist... There was'nt a "road runner" specific wiring harness meaning if you look closely you'll find 2 horn leads on the harness...One is simply not used for the road runner with a single horn...
If you really get stuck let me know and I'll get you a pic of mine..
EDIT: OK I did some homework for you and found an old thread with a good pic of the 72-up horn location...You're welcome..
LINK