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This hack is only for audio20 systems that do not support AUX inputs. Long press the CDC (CD changer) button on your audio20 and see if AUX appears on the display. If it does not, then read on. If it appears, then you should just use the AUX input. Where the AUX signal input is accessed is not treated here, sorry, there are a lot of posts about that though just follow those.
Goal: To add a blue tooth device to your audio20 system. Blue tooth device can be bought on ebay or amazon for about 20 USD and has good audio quality. 1* Cars Bluetooth Audio Cable Adapter MIC For Mercedes Benz APS NTG CD20 30/50 | eBay
Method: Follow the old hack where you solder input wires directly into CD input on the circuit board. Then put a blank CD on the CD player. Here is one of the many videos that have done the method.
YOU MUST BE FAMILIAR WITH THIS METHOD BEFORE PROCEEDING.
However since we want to input a blue tooth device instead we have to modify the method a bit.
Known problems with the direct soldering method: Audio quality suffers. This is because the internal CD player, even playing a blank CD, is still trying to inject a signal into the audio amplifier. This is contending with the signal that you are injecting (presumably from the headphone output of your smart phone, or in our case, the output of the blue tooth audio device.). Audio is weak and lacks bass. A smartphone headphone output is relatively powerful though (as it can drive headphones), so the problem might not be as evident, you can simply increase the volume. However a blue tooth device cannot do the same.
Solution: With a hobby knife or cutter, score the CD input foil path on the PCB (printed circuit board) as shown in the pictures. This will disconnect the internal CD signal completely and will allow the blue tooth device to inject its signal without hindrance and thus giving its full audio quality. Based on my judgement, the audio quality is at par with the usual CD quality. WARNING: DOING THIS WILL DISABLE THE AUDIO OF YOUR INTERNAL CD. Be ready to give up listening to CDs in your car. I do not own CDs anymore anyway. So its not a problem for me. If you plan to sell your car and the new buyer wants a CD player, you can resolder the disconnected inputs.
Here is how to do it:
Step 1: Familiarize yourself with the blue tooth device. This will have 5 conductors. 2 wires will be for power; a GND and a 12V. The other 3 wires will come together (in a shielded multi core wire), LEFT, RIGHT signal and another GND. In the pictures, the yellow wire is right, the white wire is left and the red wire is GND (this is unusual in the electronics industry). The female 3mm input jack is for a microphone input, we will not use that here. If your device came wired into a quad connector (the two square black connectors) like the one I bought, simply cut the wires off and extricate the blue tooth device. You will not need any of the quad connectors. (its wired wrong anyway for your Audio20). Try to see all the pictures first and plan where you will run the wires.
Step 2: Remove the Audio 20 head unit. There are many videos showing how to do this.
Step 3: Score the PCB CD foil paths as shown in the drawing. Be very careful.
Step 4: Solder the signal inputs as shown.
Step 5: Your blue tooth device will need a 12V supply. See the pictures where to solder the power wires. I tried to find a 12v supply that only activates when the key in in the ignition but I could not find one. The 12V supply shown is always on. This means your blue tooth is always on even if you turn off the car ignition. This will drain the car battery a bit more. The device uses around 10mA. But dont worry this is a very small amount for the car, the analog clock and other devices will draw about 50mA anyway even when ignition key is removed. Just make sure to start your car at least every 6 months.
Step 6: Test your system. If all is ok, reinstall the head unit. It might be a good idea to give the center console a good cleaning since you have so many of the pieces removed already.


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More info for electronic buffs:
The output impeadance of the internal CD player analog output pre amp will be in the low kOhms or even less. Directly connecting the blue tooth output in parallel to this output in order to reach the amplifier will load the blue tooth output considerably and cause loss of signal. I am unsure if the same problem exists if the input is from a headphone output of a smartphone, as these can drive headphones, the problem might be less evident, I haven't tried myself but some signal degradation probably occurs, especially on the frequency response. It might eventually destroy the CD pre-amp outputs after prolonged use.
The Audio20 uses a Philips SAA7706 DSP chip to manage the inputs. This chip does support an AUX input but selecting the AUX input is done through software, so unless the CPU of the Audio20 is somehow updated, its nearly impossible to select this input. It seems the later models of the Audio20 which do support Aux (long pressing the CDC button will select the input) was simply a software update. But for older models of the Audio20, it seems we wont get away with the blank CD method.
The audio quality of the chinese blue tooth devices seem to be pretty good, I cannot tell them apart from normal CDs.
 
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