Horn on my 72 just quit working.

This is probably it, but it deals more with the ignition switch:

http://www.forabodiesonly.com/mopar/showthread.php?p=1706113

Here download yourself a free 72 factory shop manual:

[ame]http://www.abodyjoe.com/pictures/Misc.%20car%20info/1972%20Plymouth%20Chassis%20Serv%20Man.pdf[/ame]

I'll admit the factory manual is not that detailed, IE removing the horn ring/ pad, etc.