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

Page 8-150

M16-20BK goes up to the column switch.

The other disconnected wire, the blue/ white is most likely switched ignition run, used for some accessory that is not installed. As cars got more and more modern, with more and more options, you'd find "more and more" unconnected connectors here and there.