The JVC KD-G421 only shows the option to change EXT IN from the CD changer to EXT IN when the current source is FM, AM or Disc . See the following page at ManualsLib:
https://www.manualslib.com/manual/81662/Jvc-Kd-G421.html?page=15#manual . Assuming the J-Link cable that you added is the KS-U57 or KD-US58, the unit is still looking for (and not finding) the CD changer. Press the SRC button repeatedly until you get to FM or AM. (Disc will only show if there is a CD in the CD slot.) Then try pressing and holding Sel. Use the fast forward/up or fast reverse/down to get to EXT IN on the display. Then turn the tuning knob until EXT IN is selected. Press Sel again to save the setting. Now see if the EXT IN is listed when you press the SRC button. Page 13, https://www.manualslib.com/manual/81662/Jvc-Kd-G421.html?page=13#manual , shows the general procedure for setting the preferred setting mode.
I hope this helps.
Cindy Wells
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