That particular diagram posted above has a mistake where the horn feeds through the bulkhead connector.
Here, download a 72 Factory shop manual:
[ame]http://www.abodyjoe.com/pictures/Misc.%20car%20info/1972%20Plymouth%20Chassis%20Serv%20Man.pdf[/ame]
wiring diagram index, page 8-144
engine compartment, 8-147
inst. panel, 8-150
On 8-147, dark green/ red tr comes from the horns to "L" of the bulkhead
Then down to 8-150, "L" of the bulkhead connector changes to S2-12Y (YELLOW) off the right of the page, THAT IS NOT CORRECT, that is the start wire from the key!!!
So on page 8-150, the actual horn wire is shown at "T" of the bulkhead, H2-16 dark green/ red tr. So I don't know whether L or T is correct
So looking at the drawing below, the pins are coded
Far left, marked K comes from the "Key" (black)
The 3 terminals at the right are marked
Top right, HS is the "horn switch" or button (black)
Lower right, H goes to horns (dark green/ red)
Center is B from battery (violet)