I retrofitted DC H10-40 with the Headsets Inc ANR unit some years back.
I also modified our RST Intercom adding a 9 V zener power supply for
the ANR headset so no battery is required to power everything. The RST
Intercom comes with a full circuit diagram (horrors!) so it was easy to
figure out all of this. The RST intercom is connected via a short
cable and a Bendix 6 pin PT series connector adjacent to the fuel
selector, to the aircraft electrical/electronic system and sits on the
cabin floor a couple of inches behind the 172M's fuel selector.