What is the function of this silver unit (letter C) by the fuse box and

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That's the horn relay and "key in" buzzer

Go here and download the 72 shop manual which should be close

http://www.forabodiesonly.com/mopar/showthread.php?p=1970088617

The horn always worked and now that you mention it, it is quiet when I put in the key....half of box does the horn and the other half does key buzzer?...might be it. Thanks for the shop manual info and thanks to all for taking the time to respond...appreciate it....Greg