I have a 97 Saturn with a Delco 12-CD changer. The pinout is like
this:

1 brown / white Left Audio Out
2 dark green / white Right Audio Out
3 black / white Audio Common
4 bare Ground / Shield

5 orange Power
6
7 black Ground
8
9
10 dark green e/c data line

I understand that I can substitute the audio from the mp3 player by
splicing into the audio lines from the CD player. Obviously, I want
to be able to switch between the two sources (the trunk CD player and
the MP3 player).

I noticed that the output from both the CD player and the MP3 player
uses THREE lines (left, right, and common). Does that mean I need a
tripole/dual throw (TPDT) switch? Or can I just ignore the common
line? Is there a signal on the common line or is it just a reference
voltage?

I was going to try a DPDT switch as shown below, but when I tried
hooking up some headphones to just the two outputs (left and right),
and it sure sounded funny. It sounded fine when I hooked up the
common line as well, leading me to believe I needed all three.


Excuse the crude drawing :-/
__________
CD player Left ----------|--x |
CD player right ----------|-------X |
| |
| |
output left---------------|--x |
output right -------------|-------x |
| |
| |
MP3 player left ---------|--x |
MP3 player right ---------|-------x |
-----------