[Mondial]

I don't know why it would make any difference if the radio was AC/DC or transformer powered as far as the switch is concerned. Its only purpose would be to mute the radio audio and replace it with the external source.

In any case its not a good idea to connect ANY external input to an AC/DC hot chassis set. You are connecting the external source to one side of the AC line. Definitely not a good idea and a real shock hazard.

Regarding your 40-155, its a transformer set so an external input not a problem. I would connect across the top and bottom of the volume control and set the radio to a band with no received signals. If you want to listen to radio, just disconnect the external source to prevent loading down the radio audio by the other source.