[glyphmon]

Okay, with the assistance (as much as I hate to admit it) of Google AI, here are the steps that I have cobbled together.  Let me know if anything looks amiss:

Parts for adding aux input:
1x SPDT switch
1x 3.5mm jack
2x 1K ohm resistors ***
1x 1:1 600 ohm audio isolation transformer

So, the plan is to have the audio switchable like I did with the 6K3.
Going to disconnect the wire running between the high end of the volume pot and the 2nd IF transformer at the volume pot terminal, route that via shielded cable out to the switch
Will route the output from the switch back to the volume pot high end also on shielded cable
The secondary of the audio isolation transformer will provide the other input to the switch
The output ground of the transformer will run into the chassis and attach to the B- ***

For the jack, the left and right channels will be combined via 1K *** ohm resistors, the combination of which will go to the isolation transformer primary input
Ground from the jack will go to the transformer primary ground
All of the components will be mounted on some combination of a PCB base and/or plastic project box onto the non-metallic rear cover of the radio

Now, the asterisks above represent clarifying questions:
Resistor value: for the RCA 6K3, this was accomplished with 100 ohm resistors; here Google has suggested 1K ohm; is the difference because this is an AC/DC radio; because of where and how the signal is being introduced into the radio - the 6K3 utilized an existing phono input; or, not actually different at all and I'd be good with 100 ohms here as before?

Ground from transformer to B-: does this need to be shielded cable as well? and it shouldn't matter where it reconnects, as long as it's to a B- point and not the chassis?

I'm also going to add a polarized plug end to the cord and probably will try to relocate the switch to the hot side, though that's going to probably require adding a terminal strip somewhere - there's a lot going on on the back end of that power switch.  Will also plan to add a 0.5A fuse before the switch.

Also, I should have asked this probably sooner, but where C11 and C12 (0.05uF each) connect to the line and B- and B- to chassis respectively, should either of both of those be safety capacitors?  Same value as original?

Schematic attached.

Best wishes,
Roy