Citronnie

Hi I have got a Xsara which I have fitted a Clarion CD player to. I now want to fit a CD changer nut I am not sure which one to buy. I will probably get one from E-bay and wanted to know if anyone knew how they should be fitted and which model. Is the cable already fitted because when I fitted the stereo there was not a spare socket on the unit. Where are they usually fitted? Any help will be much appreciated. Thanks Citronnie