As you indicated, the mono and stereo paths from the radio is dictated by the correct radio to tape player harness irrespective whether the radio is mono or stereo. I've attached some diagrams from a 1971 Delco Radio service Manual and some colored diagrams from a '67 T200 and mono radio to illustrate how a mono signal is switched through the harness to the fader L-pad within the tape player to drive the signal to front and back. When a tape is inserted, the mono path is severed and each stereo channel from the tape player is sent to a speaker.
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