General car wiring, not Cobra related

[kayjayd]

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

[KevinW]

'Common' means 0 Volts IMHO, but its a 'common' for that part of the loom -
may or may not connect directly to earth/chassis, IMHO.

black would be my guess for 0 Volts.

Others probably more knowledgable than me. No C-Max forum available?