So after a few Google searches and many hours of trying to find something I understand now that there isn't a simple cable that can be plugged into the CD Changer input (13P DIN socket) of the Infinity head unit on my 2000 Galant.
When connected to a CD changer, there's a protocol that goes on between the head unit and the changer that enables the audio in and allows the buttons on head unit to control the changer. So the only way to enable iPod (or any other MP3 player) through this input would be to simulate the CDC protocol.
The best link I found so far is this:
CD Changer Interface & ProtocolsThere's a link to one interface that's an 8P DIN but not for a 13P DIN. The other interesting thing on that page is that the Pioneer 13P DIN pinouts seem to be the same as the Matsushita. The Kenwood 13P DIN does not match at all.
So for now it looks like the splice-into-the-pre-out-cable posted in another thread seems to be the best idea, but I can't bring myself to splice it!!! I'm thinking of adding a male-female pair, where the male plugs into the head unit, that connects to a female where the factory cable plugs in, and in-between the two I'd splice in the cable from the iPod.