horn not working

"Simple." LOL

Do you have a good wiring diagram, preferably a factory service manual? Go to MyMopar and download one. You may have to settle for Plymouth as opposed to Dodge.

http://www.mymopar.com/index.php?pid=31

Also at MyMopar are these SIMPLIFIED diagrams. These often have options left out, connectors left out, and are not OEM Mopar. But sometimes, they are easier to follow

http://www.mymopar.com/downloads/1971/71ChallengerA.JPG

http://www.mymopar.com/downloads/1971/71ChallengerB.JPG

Start by accessing the connector coming out of the steering column, the signal connector. The horn is often a black wire. You can confirm that by measuring power at what you suspect is the horn button wire.........it should be hot at all times, then yank out the horn relay, and that wire should go dead

Once you find that GROUND it, and listen. You should be able to hear the horn relay "click" whether the horns work or not

EDIT just looked at the diagram, and it indeed should be a black wire at one end of the TS switch connector there, coming out of the column

If you hear the relay click, "move" your meter / test lamp out to the horn wire and see if the relay is supplying power.

The relay in your car, by the way, should be plugged into the fuse panel / bulkhead connector assembly if I recall

Likely trouble in no particular order:

Horn button, TS switch harness connector, bad or missing relay, fuses, bad connection through the bulkhead connector, or bad horns or not grounded.