Bill sometimes when the stars are out and the wife is down below Janis
Joplin or Mozart needs to be heard Loud with disturbing others......
As for your directions I understand the premis just a bit confused with
the connections.

You might also consider some wireless headphones. They usually have a base
station of some sort that could be installed below decks with the radio and
controlled via a switch. Most 900mhz or 2.4ghz wireless headphones have
reasonable enough power to carry 20 feet or so. That way you'd avoid having
a plug up on the helm and a wire tangling you. Wireless headphones usually
want to use line-level signals. You'd want to split that out from the head
unit.

As for connections it should be simple enough. But it will depend on how
your stuff is actually wired. If the speaker outputs on the head unit
aren't being used then you'd need to make the wiring. You'll probably have
to make the wiring anyway, so if you're not comfortable with this then just
find a local installer.