Ok here is what I did on a GM/AC/Delco made Radio on my Toyota Matrix (this radio is similar to most radio on GM cars/trucks)
- Trace the audio signal from the CD player to the amplifier board by using head phone. Cut the plug to expose bare wire or solder/tape 2 piece of wire (GROUND and LEFT or RIGHT) to the plug and use it to probe around.
I found 2 signal wire on a 6 pins connector (Black pin 1 is LEFT and white pin 3 is RIGHt pin 2 is GROUND).

I cut that 2 audio wires pin 1 and 3.

Solder a DPDT relay

The result is a mini stereo jack for aux in and a wire dangling out side to enable and disable aux input (driven the relay)
There is NO noise or engine whine when I use a discman to feed the audio. No computer setup in the Matrix yet so I don't know if there will be noise with the PC.