What you need:
-- Stock car stereo with a CD changer input
-- Basic Soldering Skills
-- Soldering Iron & Solder
-- Hand held Volt/Multimeter (to find right wires)
-- Small gauge speaker wire (anything around 18-14 gauge will work)
-- 2 panel mount 1/8" stereo phone jacks ($2.99 for 2 at Radio shack)
-- 1/8" stereo plug cable ($5.99 at Radio Shack)
-- Old pair of headphones (really just need the wire & 1/8" plug)
Essentially what you are doing is sending a pirated signal from the MP3 player onto the line that would normally be sending audio from the CD changer. In order to prevent a mixing of the audio from the changer & your new pirate Aux input, you have to play a special CD which contains tracks with perfect silence. It may sound complex, but is quite simple with some basic electronics/soldering skills.
Step 1: Remove Your Stock Radio From the Car
You may also try calling local car audio shops and ask them for tips on how to remove the radio from your specific make/model. Most are moderately friendly and will give you tips, just don't expect them to come do it for free or anything.
Step 2: Locate the CD Changer Plug & Pins
Now here's where you have to do a bit of googling. Figure out what the pin layout is for your radio's CD changer. Try searching with something like "Honda Accord 1996 CD changer pinout" I was able to find this layout for an Acura which was close to my Honda for the basis of mine. Remember, there's only a couple car makers, so there's a lot of similar equipment out there across brands. Honda Civic may be close to the Accord, and also close to an Acura. The same should apply for domestic cars made by the same manufacturer.
The 3 pins you need are:
Step 3: Solder in Aux Input Jack
Step 4: Mount the External Jack in Dash
Use the old headphone cable and cut to the appropriate length to connect the internal and external jacks. Plug the cable into the internal jack, and solder the wire ends to the external jack. Take caution to match the pins (refer to plug layout diagram if needed) This cable serves as the quick-disconnect patch cable, should you need to remove your stereo or take apart your dash in the future. Soldering directly without this patch cable could be a pain later if you ever need to remove the dash or stereo for repairs, etc.
When drilling in the dash, be very careful and know exactly what's behind your drill bit so you don't accidentally drill into something important!
On mine, I had a spacer/junk bin that was right below the radio. This made an excellent location to mount the external jack since it was inconspicuous and also I could wire it up at my workbench instead of in the car. Choose the location you like best to mount the jack. Push the jack through your hole you drilled and use the nut supplied with the jack to screw onto the jack and hold it in place.
Step 5: Test Your Connections & Reinstall the Radio.
Once it's all together, all you have to do is put the CD changer on your "silent" CD, put it on track repeat and use the MP3 player to pump the tunes.
You can see here's my setup with the jack installed. Sleek and WAY better than an FM transmitter.
Step 6: Burn Your Special "Silent" CD
In order to do this we just burn a special "silent" CD. It's got regular tracks, but there's no sound played. Download the 5 minute "silent" MP3 below and create a CD using your favorite CD burning software. I took the same file and added it multiple times to the CD so that I had many tracks. Just fill up the CD with these blank 5 minute tracks so even if you don't have the CD on repeat, you'll still hear nothing from the CD and everything from the MP3 player.
Once you've burned the CD, pop it in your changer, put it on repeat play, and you're all set. Turn on your MP3 player and test it out.
At a cost of under $10 and 1-2 hours (depending mostly on how easily you can get your radio out of the dash) it is well worth the great audio you'll now enjoy!