Didn't figure out how to remove the cushion, and the magnet fishing expedition went badly, there's a LOT of metal in there that just sucks the magnet to it. I tested with the other earbud and while it'd stick to the magnet, the amount of plastic made the connection not so strong.
However, a feature I never knew existed and can't figure out when I'd ever need it besides now, is the little button on the door panel that lets the seat lift up from the rear and slide forward to let someone get in the back the hard way. Anyway, when the seat lifted up, the earbud was right there for me to grab. Son is happy now, he can listen in stereo again, no more mono.
Thanks
@Dth2Chrm for the suggestions.