This is a mod for a year 2000 audi with the symphony 1 head unit, however it will work on all years of symphony 1.
If you have a symphony 2 this will not work there is a different way to do it.
Sure you could use a tape adapter but I for one am sick of hearing the gears moving inside it and don't like sticking my finger in the tape player to move the tape around to find the perfect spot.
With this mod you will loose the cd player, there is a work around to have a switched input to still be able to use the cd player but I do not describe that here. You may have seen other mods that utilize the cd changer in the trunk this is not that mod, it requires almost no extra equipment at all.
What we will be doing is locating the right and left analog audio output coming from the cd player and injecting our own audio feed.
In the end for this to work you will need to have a cd in the drive to fool the processor into thinking there is a disc but we will feed it our own external source of audio.
This writeup is assuming you have knowledge of electrical testing and soldering.
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Step 1: Materials
I used a panel mount aux jack like this one.
However you can sacrifice a pair of headphones for the jack if you don't want to use an input jack like I did, if you go that route just strip off the jacket to expose the sets of wires we are looking for. Use a multimeter on continuity to find the right wires; the tip of the jack is left, the center is right, and the back is ground.
Step 2: Remove the Face of the Unit First.
Remove the side screws as shown in red. Be careful there are connectors that go between the boards.
Step 3: Remove the Top.
Screws in red then just pry the top off gently.
Step 4: Remove the Back.
Screws in red, be careful when prying this one off, it has connectors that go between the boards as well.
Step 5: Remove the Cd Player.
Remove the 2 screws on each side and unplug the ribbon cable that goes to it.
Step 6: Locating the Pins and Soldering to Them.
This is the hardest part. In the upper left corner you will see a breakout board that plugs into the main board retained by one screw and a bent metal tab.
Remove the screw and un-bend the metal tab then gently unplug the little board.
Next we need to cut the wires in the plug to permanently disconnect the cd player's audio output. We do this because we don't want the output audio from the cd player to be blending with the external audio we are injecting.
Referring to the images of the black connector the best way I can describe it is to orient the board exactly as you see me holding it. In the first image with the board facing left you need to cut the bottom most lead then push the lead to the side so they aren't touching anymore.
In the second image with the board facing right you need to cut the lead second from the bottom and push it aside as well.
If this is too confusing let me know ill try to get better pics/ describe it better.
If you are using the end off some headphones be sure to check the next step on drilling a hole in the back to run your wire through before you solder the wires to the board.
With that done now we can solder on our new leads to the aux jack, I used blue, red, and black wire. Its not important what color you use just be sure that they go to the correct pin. The last 2 images are the pins that are used. On the top row of pins only, the first is right input, the second is left input, the third is ground. This is not to be grounded to your car but rather to your headphone jack common ground. Be sure of course that when you are done soldering that nothing is shorted together.
Step 7: Drill a Hole in the Back Plate.
Drill a hole in the back plate to mount the input jack to, or if you used the end off of head phones drill this hole then run your jack though before you solder to the board.
The jack I used required a 1/4" hole.
Make sure not to leave any metal shavings behind, they could get stuck somewhere and cause a short.
Step 8: Put Everything Back Together.
When you put the back plate on be sure not to pinch any wires.
To use your new aux input just stick a cd in, if you did it right you will not hear the cd play at all. Now plug your device in and enjoy.
I opted to use audacity to burn a cd with only one track on it that consisted of silence, go to generate > silence, input however much time may be needed to fill the whole cd. Something like an hour or so long.
If you use just any music cd, once the track ends you will get a momentary cut out of your external source until the next track starts playing.