Horn on my 72 just quit working.

Your 72 horn wiring is totally different than my earlier Mopars (64, 65, 69). Good work 67Dart273 on finding the schematic mistakes. I also traced it thru the T terminal to the starter relay and said "WTF?". Somebody mixed up the "DK GREEN RED" and "DK GREEN RED TR" wires. Anyway, sounds like the problem is in the wiring or switch in the steering column.

If like my cars, all the horn button does is ground the black wire. It could be in the button itself or in the wiring/connector to the steering column. First check at the connector (easy). If OK there, you will need to pull the steering wheel. There is a good tutorial on your car w/ photos posted on FABO. Needed, since your in-column key switch with steering lock is pretty complicated.

Finally, why did Chrysler integrate the horn and buzzer? Must be some dubious feature like "don't sound buzzer while horn is blowing".