Here is my aux input write up into a factory CD player. It is based off of reconjb's write up. On this build, I'll be using a 6' stereo extension cable from wal-mart (Belkin PM0811 / Wal-mart SKU 7 22868 67667 7).
I cut the 3.5mm jack off about 4" from the tip.
I then split the ends on the long side with the 3.5mm plug on it.
I used a 12g wire cutter just to make the cut for the main cable then pull the black sheilding off by hand.
As for the red and white cables inside, I used the 20/22g to make the cut and again, my hands to pull the shielding off.
Red = Right
White = Left / Mono
Unshielded = Ground
To pull the radio out, I used Scosche DIN radio removal tools (DT1 / Wal-mart SKU 0 33991 42011 3).
More to come tomorrow as I get into this more...