Download the JD chassis manual from my server, http://www.cdr-roc.com/89JD/
. Turn to page 40--30-42. That is where you will find a description of the circuits and related diagrams. A volt meter will be your friend to help you chase down the problems.
We have had horns go bad due to water getting in them. The other issue we have had is corrosion at some electrical connections. The weird ones can test good but when the connection needs to carry a current it no longer works.
Feel free to download any of the info in that folder.