So I found a way to add an auxiliary input to my factory radio/cassette player. Originally I was going to tap into the cassette and just pop in a dummy tape when I wanted to play it, but my cassette player doesn't work, and doesn't recognize a tape is inserted, so I can't use that.
I started probing solder points with a chopped up headphone cord, attached to some over the ear headphones. Eventually I found several spots on the board that played music through my headphones. I traced these back to a IC on the circuit board. There are four pins on this IC that play audio, two are lower volume, and two are a bit higher. My guess is that this is a pre amplifier, and one input is the low power radio source, and the other is the higher output that goes to the main amplifier.
I tried tapping into the higher output pins and was able to play music from my phone. There was popping sounds when turning up the volume, and it didn't sound right. Next I tried hooking up to the lower output pins. The output from my phone was way too much for it and it caused a lot of distortion. However I noticed that when the leads we're held in my fingers, it played perfectly (very high resistance). So I'm going to pick up a potentiometer in the morning and hook it up to the lower output pins and see if it works. I also cut the traces on both leads which I will hookup to some switches so I can cut all Audio to from the radio so there is no bleed over.
It seems like this could be done on any of our factory radios, just probe the pins on some of the IC's with it hooked up to headphones or a small speaker while the radio is playing. Once you find the pins, you know where to tap into. You could add a simple headphone jack from radio shack that cuts out the radio when it's plugged in and only plays from you auxiliary source. I'll post up more when I figure it all out.
Here is the IC I used. I marked the pins with a sharpie that I ended up using. I soldered into the back side, and cut the traces with an exact knife on the top side.