DIY AUX unit with steering wheel controls

On my previous car, I hacked together an aux input with two CD-ROM plugs as described in many places on the Internet (here's one). I was really happy with it, and fully intend to do the same on my new (?) second-hand car, which also has CD-changer support, but no AUX. (Neither car had a CD changer installed, because I couldn't be bothered).

The new car has steering wheel controls though, and I thought this could be a nice upgrade, but I'm really not sure how to do it.

The pin-out diagrams for the CD changer plug (which I stole from here) for the CN702 12 pin connector shows lines 9 and 10 as TX signal lines. Since there are track (and disc) up and down buttons on the radio, I assume this is how the commands get through to the CD changer. And the phone obviously has support for headsets with track skip buttons. So the stars seem to align.

Except for the details. 

First, a normal audio cable only has three wires and one with headset controls have four: Ground, left, right and comms. I think I can connect the following pins on the 12 pin connector to ground: 6, 7 and 9 and then 1, 2 and 10 get connected to the correct wires. Is this correct?

Second, I have no idea if the actual signals sent to TX for a CD changer bear any resemblance to what the phone expects from a headset. Does anyone know? I can probably hack a test using a bread board. I think.

Third: I really only dabble in these things. Did I miss anything really important?

Picture of DIY AUX unit with steering wheel controls

TX in 99.9% of cases means you need a digital signal to do things - basically everything the unit can has a corresponding TX code to be used.

bmiller911 year ago

Slightly confused by what you're trying to accomplish. Just adding aux audio input but want head unit to automatically recognize that aux has been connected and switch to "CD" or pass commands from steering wheel (Skip, pause etc. to external device e.g. phone?