First, of course, i took the radio out of the car. Ford wants you to buy some stupid tool to get it out, but instead i used a clothes hanger :P. Basically, you push some long metal rods into the 1/8in holes(four of them, but you can do one side at a time, so two at a time.) on the front of the radio and this pushes in the springs and releases the radio.
On the back it has two harnesses and one-wire interface that runs to Volume, Preset, and Seek buttons next to the steering wheel(very useful, I think :)). One harness is for the power and other stuff like that, and the other goes to the speakers(the amp is built into the radio).
To take the top cover off of the radio, it was just some Torx screw and i easily got them out with this handy tool set.
Taking out the tape deck to get to the PCB.
Some nice little solenoids and a motor in there.
Radio top PCB
Radio bottom PCB
Audio amps on the heatsink
The Radio IC decodes the FM signal to audio, i think. Tape decoding IC does similar. Mixer IC takes the Tape and FM audio signals and switches between them/adds EQ/balance/fade etc. The main IC controls all of the other ICs/LCD/buttons/
Now comes the real reverse engineering. At first i was going to find the tape audio line, as it didn't work anyways, and inject aux audio there. As i was trying to find this tape audio line, i found it very difficult; I had narrowed it down to a few wires coming form the tape IC area going to the mixer, but i didn't know for sure. So i connected a little speaker(GND and the other wire free) and probed around on the mixer IC to find the audio lines. With this method i found both the FM and tape audio lines. Since the volume is adjusted by the buttons in the mixer IC and not in the tape decoding area on the PCB, i cut the tape audio trace and soldered in the headphone jack. If i didn't cut the trace, the mixer IC would receive noise from the tape decoding ICs and the aux audio at the same time which would be bad. I threw in a tape(so the radio changes to that channel in the mixer IC) and hoped for the best. Surprise, it didn't work very well! The audio was only playing on the left side, and sounded really distorted. After i thought about it, i found it strange that i only found one input line from the tape ICs to the mixer chip when it is stereo audio. I went back to the PCB and found another trace that ran form the tape ICs to the mixer, but when i played a tape, this line pure noise; strange, right? I went through all the pins on the mixer IC with the little speaker and i could still only find one audio channel for both the tape and FM that sounded like audio and not distortions. Maybe the tape and FM ICs put out a LR mixed audio line that is later split? I dunno. So that plan didn't work.
Next i though maybe i should bypass the mixer IC all together and inject the aux audio into the amps directly. I could turn the radio volume all the way down so nothing was coming out of the mixer IC(since that is where the volume is adjusted) to the the amps and so the aux input shouldn't have any noise issues. But first i had to find the audio line inputs to the amps.
I knew that harness A(look at the pic below) was for power etc, so mostly ignored that, but harness B is for the speakers. Every other pin on B is GND and between the GNDs are the speaker pins. From looking inside i already knew where the speaker amps were, but i couldn't find ANY info on them because they are propitiatory. So i had to use a DMM(multimeter) and its continuity function to trace backwards. I forgot that i cant test continuity through capacitors, so that plan didn't work until i tested for continuity until the caps, and then jumped to the other side of the cap and continued testing. I drew a little pinout on paper of the IC and used process of elimination to find the audio inputs to the amps. I started by using harness B to find the out put pins on the amp ICs and crossing those out on my pinout, and then i found GND, VCC, and any other pins that were linked together(if they were linked together, it wouldn't make sense that they were inputs, would they?) In the end, it was pretty obvious; four small traces(for each speaker) running to the mixer IC.
Since the audio jacks on pretty much any Mp3 player has only left and right, i soldered the front/back left speaker amp inputs together and connected that to the left pin of the audio jack and did the same for the right. I turned down the volume to 0 on the radio, connected the iPod to the audio jack and it worked! Crisp, clean and on demand music! To use the FM radio, i just unplug the iPod and turn the volume back up on the radio
But, how do i get these voltages? A voltage divider, of course. Below, V1 is 5v, 0V is GND, V2 is the output voltage, in my case. R1 and R2 are resistors and depending on their values/ratio, the voltage output on V2 varies. This is a great calculator for finding what resistors you need. i found that with R1 being 68kohms and R2 being 47kohms, i get 2v on V2. With R1 being 22kohms and R2 being 27kohms, i get 2.75v on V2. But the radio voltage wasn't exactly 5c; more like 4.91, so that changes the values a bit.
One thing that i found strange was that the voltage from the divider on both pins was .2v lower than the calculator said it would be. Even though the Radio was putting out 4.91v, that still isnt enough to drop the voltage .2v. So i choose resistors that would give me 3v theoretically on D-(2.8v really) and 2.2v on D+(2v really). It worked great; I got almost exactly 2.8 and 2v.
Adding in the USB and audio jack.
Used some Sugru to hold the connectors in.
Still, i have one last issue. If i plug in both the charger and the audio cable in at once, i get this strange ticking sound through the speakers, but not when i have just the audio cable. Maybe add a cap to the usb plug? But, overall, this mod worked out quite well!