Originally I had planned to just buy a new after market head unit for my car, do to its lack of an AXU input for my iPod, however after seeing nothing within my price range that was decent looking, I realized that I may have been on the wrong track. While doing some research, I stumbled upon this forum posting about adding an AUX input to an old car tape deck. After digging some more on this topic I found that people have been successful with hijacking both the AM/FM and CD input on stereo circuits. The CD methods seemed like a bad idea due to needing a silent cd to play constantly, which is just asking to wear out the stereo cd drive. So I went with hijacking the FM audio out pins of the stereo. The interesting part about using the AM/FM method is that due to the Auto Gain Control built in, when a MP3 player is connected, its stronger signal seems to overrides the AM/FM on the board and effectively shuts it off without any extra effort on your part. From what I have read, this seems to be the case with most car stereos, however your results may vary.
Here is a good place for the standard warning: I did this project knowing it could potentially destroy the stereo and/or the mp3 player, if you attempt the same thing or something similar you face the same risks. Do not blame me if you break anything by following my information or pictures, I express no warrantee on this information's correctness or usability.
I started off my taking the stereo apart and looking up the chips on the reverse side of the board shown here. There is a lot of mixed opinion on where the best place to connect to is, and it seems to vary from different stereos, so it is best to see what you are working with first. While this gave me some good extra info, I quickly realized that this was not necessary, due to the pinout being printed on the back of the board. I did some voltage checks while the stereo was plugged in and running, the FM outs gave a normal voltage variation between 0v and 3.6v, which means that I would not overload anything with a mp3 player's small voltage output. After figuring out on paper what I wanted to do and finishing my voltage checks, I did a quick test run with a temporary 1/8 inch plug connected and an older expendable mp3 player. While I was expecting this to work out, it was exciting to hear it function, when the player was plugged in, the FM cut out and the mp3 was clearly playing unplug it and the FM comes back as normal. While the signal is stronger, the max voltage from the mp3 player is lower than the FM, so the mp3 player needs to be set to at least half volume to be heard well, not bad, but definitely not perfect. There may be a better connect point that uses a lesser voltage, however I am sticking to the ones on the edge due to it being easy to access and solder.
After testing and finding out that this did indeed work, I drilled a hole in the back of the stereo and added a 1/8 inch plug and then ran a short shielded audio cable to the pins I used in my test. The board picture above has the final wiring, which pokes out of the side and is soldered to the correct pins coming from the separate AM/FM board and ground which is a larger solder point connecting to the metal housing. By using a shielded cable inside the radio, it will prevent any extra interference due to the wire stretching across the board from the back panel. After doing one final check, I reassembled the stereo, now with its new AUX plug. Finally, I put together a separate cable to plug into this AUX port and run it to a more convent location in the center console.
The wire runs down from the back of the stereo and connects to another 1/8 inch plug from which I can easily plug in an mp3 player or another other audio device. After making sure everything was in order I snapped all the console panels back into place leaving just the new 1/8 inch port left in view. This is nice, because it means that there is no obvious indication that the stereo is no longer stock, making it a far less likely target for theft, not that the radio is easy to get out to begin with.
After doing some actual road testing and letting it run for a while using my expendable mp3 player, I am quite satisfied as well as confident that there is no chance it will harm my iPod, so now I am ready for the commute on monday morning. I must say that the only down side to this setup is that the mp3 volume needs to be over 50% and the stereo volume ends up being about 12 higher than cd or radio (25-30 vs 10-15 out of 40). Not that this is a problem, it is just the only flaw to the design and may be fixed by using a closer point to the volume control chip. Either way, I am happy and my car now has a handy AUX port, all at the cost of $3 for 1/8 inch plugs.
Other possible options instead of this and costs...
DIY AUX ($0-$50 depending on your extra parts supply)
Clarion CeNET adaptor EA1251B ($99 and cannot have CD Changer option installed)
New separate head unit with AUX ($60-$500)
