I bought my wife a Ford CMAX the other week and I want to wire in a 3.5mm audio jack to the AUX button on the stereo. There's detailed instructions on this link -
Wiring a C-Max Audio Aux Input with a Visteon 6000cd 
However the part of the quad connector that's mentioned in the instructions isn't fitted in her car. I need to wire AUX left and right to the 3.5mm jack that I've bought and then supply a "common" from the loom. In the context of the instructions what does he mean by common? Live, Earth? Also if it's live or earth what colour wire would I need to tap in to?
I'm sure one you clever lot will know the answer
Thanks in advance