That looks like it could work. It seems like that would provide line level output, would I need a separate potentiometer for the headphones or would the head unit suffice for it? If I do need one, would this work?

Regardless, I'd need a way to switch the output between speaker and headphones, so I'd need a switch wired in between the head unit and the Kicker adapter you linked, that could send the signal to either the speakers, or through this RCA adapter. All I can see are RCA or 3.5mm versions.

Thinking about it now, the head unit has 4 sets of outputs, 2 for front, 2 for rear. Since the front and rear output would be the same, I can wire one set to the Kicker adapter, and the other set to the speakers, with a switch in between the speakers to cut the circuit when using headphones.

Actually, the deck has FAD/BAL knobs on the front, so theoretically, I can just wire both of them hot all the time and just fade out the either the front or rear outputs at the twist of a knob. I think I'm ready to start making it soon.

Thanks!