You were on the right track to install a phono jack. The manufacturer would have installed a switch to select between radio and phono, but you could get around that by using a closed-circuit 1/8" miniature phone jack as in the circuit below. For radio operation, the detected signal comes to the jack on the green wire and then to the volume control on the red one. When an external audio source is plugged into the jack, the jack opens the green radio circuit and sends the audio signal via the red wire to the volume control.
Back in the day, nobody bothered about making the phono connection idiot-proof and so on AC/DC sets the shield connection of the jack could have been connected to chassis ground as shown and therefore "hot" depending on which way the power plug was in the receptacle. Today you'd want to put an .05 or so mfd. cap between the shield connection of the jack and chassis ground, the jack being insulated from the radio chassis in some way. I'd also run audio wiring like this in shielded cable under the chassis, grounding the shields.
Reece
