For manuals, parts books, and other things "1965 - 1955",
www.jholst.net This is primarily a 300 Letter Car oriented website, but the parts books and such are the same, letter or non-letter car.
Getting the rear seat cushion out can be somewhat variable, by observation. A simple retention device that can take some seemingly-major muscle to get things to unlatch and latch again. BUT, once you get the hang of it, not too bad. BEFORE you put the cushion back in, make sure of where any seat belts are!
As I recall, a slight pull of the cushion forward, then a quick depression and simultaneous push rearward usually does it. Two "latches", about in the center of the rear footwells Do one side at a time. Might need to put a towel over any cloth you might have to apply pressure to, for good measure.
In theory, there might be a build sheet on EACH of the interior seats, rather than just the rear cushion. That's just my theory and no more.
I believe that the '65 and '66 radios will interchange, as the instrument panel is basically identical in those two model years. I believe the power antenna mounted on the rh rr quarter panel, but also needs an instrument panel switch to run it up and down (on the opposite side from the map light switch, in the map light's bezel), I believe. So . . . antenna unit, switch, map light bezel with switch (or possibly carefully cutting the bezel itself), related wiring harness.
Personally, the power antenna location looks a bit "strange" to me, compared to the location of the normal front antenna. That was not a really big option, back then. Would probably need to completely overhaul any used antenna mechanism you might find?
Back then, FM radio was generally only in the larger metro areas, especially range-wise. But then we didn't have nearly so many 100K watt stations. Which might diminish the availability of OEM AM/FM radios themselves. The AM radios, from my experience, were very good in reception and sound quality. I would suspect the FM radios to be similar in those respects.
It is EASY to add an additional rear speaker, provided the rear seat heater option is not there. Back when it was a "used car", I added a factory OEM rear speaker to the existing one in our '66 Newport Town Sedan. Used a Chrysler accessory kit to do it. Very much worth the effort!
As you've probably discovered, the reverb option can be "neat" or quite the opposite, depending upon how you adjust the delay.
So, while you're waiting to find a AM/FM radio, you can add the 2nd rear seat speaker. You might look around for pictures of '65-'66 Chryslers with the rear power antenna option so you might make a better decision of if it would be worth it to do (especially since the coax would go from the rear quarter to the instrument panel and such).
That is ONE BEAUTIFUL New Yorker!
Enjoy!
CBODY67