Play audio from raspberry pi through my car's disc changer
As noted in the comments, most modern car systems use a digital protocol to control the CD changer.   Even if there are analog audio leads, the head unit won't recognize it without digital signals from the CD unit.

In any case, the solution will be specific to your car and possibly the specific head unit.  If you search (Google, etc.) for your car's model and terms like "add aux input", you may find more detailed info.   You  may get lucky and be able to use a fairly inexpensive adaptor.