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Some thoughts: If one radio is stereo the other mono the connector may have different pinouts on the different sets.
It certainly looks like the last pictured connector is mono. That reply about resistor summing of stereo to mono is valid...If you short two channels of signal source together performance of both channels may suffer (possibly resulting in damage to the source hardware in the worst case).
Other possibilities in the variance in input audio quality (presumably relative to comparable radio sound performance) may be in the individual radios input circuits/expected input devices. A phono input might be RIAA equalized, a tape input could be a flat line input, or it might have an equalization for direct tape head line in.
Some European connectors have odd pinouts and or redundant grounds. If feasible I recommend getting the schematic and tracing the connector out to confirm you are wiring it correctly...If you are wiring signal ground of one thing to hot of another and vice versa and both signal grounds are coupling through the power systems things can get weird.
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