Optimus 120-1998 - Manual (Page 8)

Stereo Cassette (120-1998) Compact Disk Use Faxback Doc. # 6928
CONNECTING A COMPACT DISC PLAYER
You can connect a portable CD player (or other audio component with a
line-level output) to the stereo.
Use a stereo mini-jack cable with 1/8-inch plugs, such as Cat. No.
420-2387. Connect one end of the cable to the stereo's CD IN jack.
Connect the cable's other end to the output jack of the CD player when
you are not using it.