How to invert one channel of phone output?
Regarding the electrical interface you need stereo sound so try to use the CD inputs.


C18: GND.
C19: Left.
C20: Right.


Get a cheap pair of headphones or a 3.5 mm stereo jack extension lead and cut the headphones off. Connect the tip to C19, ring to C20 and sleeve to C18. (Use your multimeter continuity tester to identify the cores.) Try it out and if it works then do a decent permanent job. You may be able to find a push-on spade receptacle at a motor parts supplier.