My car came with a Blaupunkt 520 as the factory fit head unit. As it uses the on-steering-wheel controls, I don't really want to swap it out, but I do want to add the ability to plug a device into it such as a phone. The car had a CD stacker option, so my original idea was to find the inputs they would have used and add a 3.5mm jack.

After pulling the head unit out, I found the below sticker with the pin outs. Most I can figure out - FL, FR etc are the speakers, IGN is main power, 12V BATT is the memory, ILLUM is back-lighting and Earth is obvious. There are some there I'm trying to figure.

Using the information I have, does anyone know how to add an AUX input to this stereo head unit? I presume I need the TEL-AUX and maybe the TEL-MUTE. I have no idea what SDA, SCL, SDVC, MRQ, DIAG or SWC represent. I presume A-GND is an earth of some kind but not sure what specifically.

The first image shows the sticker on top of the head unit. The second is the receptacle in the car. You can see where some of the pins are not yet in use.

Stereo pin-out label

Mating receptacle

  • \$\begingroup\$ Are you sure that the changer didn't transfer digitally? that the cd stacker option did not also include a different radio? you might want to locate informations about the changer (pinout etc.) \$\endgroup\$
    – PlasmaHH
    Feb 18, 2015 at 11:07
  • \$\begingroup\$ I never asked at the time, and it was 10 years ago. I have searched Google, but my Google-Foo is failing me. \$\endgroup\$ Feb 18, 2015 at 11:19
  • \$\begingroup\$ I just had a quick flick through the user manual for the head unit. There is a paragraph about the hands free feature (if fitted). The manual suggests that a hands free capability is possible with this model unit. \$\endgroup\$ Feb 18, 2015 at 13:05
  • \$\begingroup\$ telaux. usually mono. \$\endgroup\$
    – PlasmaHH
    Feb 18, 2015 at 13:05
  • \$\begingroup\$ If you want to do it the easy way, there are kits for most cars that let you replace the head unit and keep the steering wheel controls. \$\endgroup\$
    – Grant
    Feb 18, 2015 at 14:49

2 Answers 2


Pretty much most CD changer options on factory radios require digital communication to enable. SCL and SDA typically refer to I2C clock and data lines. Sdvc is possible voice control, swc is likely to be the wheel control. The middle eight pins are the four speaker outputs. Diag is diagnosis, mrq is likely to be an interrupt input? A ground could mean aux or analog ground.

That said, the picture of the connection harness shows its not wired for the voice option. Analog or aux ground, tel mute and tel aux have no contacts. The radio may support it but the wiring for it does not

  • \$\begingroup\$ I did notice those specific contacts missing. I hope to add them, I'd just like to get some guidance before I make an incorrect connection and blow something up. \$\endgroup\$ Feb 19, 2015 at 4:03

After a bit of trial and error, I figured it out. enter image description here

Referring to the block diagram above, I figured out the TEL-AUX is a mono input and the TEL-MUTE is a trigger that switches the head unit to PHONE mode when an earth is applied.

Describing the circuit:

  • A 3.5mm jack on the dash allows me to plug my phone, MP3 player or any such device in. This stereo signal has the L and R channels shorted and pass to the head unit. The common is connected to the same earth as the head unit earth. I tried other earths, however this is the cleanest earth with little or no interference from the alternator.

  • A relay is the core of the circuit. It applies an earth to the head unit TEL-MUTE input to tell the head unit to switch to PHONE mode. It is powered from the accessories power to the head unit via a switch on the dash. This allows me to select PHONE mode manually when I want, and off lets the head unit go back to normal operation. When the key is not in accessories the circuit powers off automatically, preventing discharging the car battery through the relay. Additionally, I found that my head unit remained on even with accessories off whenever it was set to PHONE mode.

  • Connections into the existing ground and ignition wires were simple solder splicings. Jacking into the previously non-existent TEL-MUTE and TEL-AUX was the tricky part. I couldn’t identify what contacts the molex connector used, so I made a spade-like end by folding the wires over and jammed them in. The interference fit has been holding well for the last few days and if I ever need to remove the head unit, I can as it is not hard wired in (as I was contemplating soldering them).


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.