If you are looking for one, I bought one from Crutchfield.com for my 2000 S40, I had to run a new line (supplied with changer) from the trunk to the back of the stereo because the factory cable and the aftermarket cable have different connections at the changer. Once I got it to the back of the stereo, it plugged right in. It works flawlessly.
I have a 2001 S40 and they have the cable chased from the boot(or as yanks call it the trunk) to the back of the head unit. You need to pull the head unit out, untape the lead and plug it in (asuming you have the double din hu- unit 605-655 most common) it will say on the front of the unit. You do need to but the fitting kit Which gives you a power lead you just feed off the electric aerial and the mounting brackets. The only CD changer that I know will work is the 10 Disc Volvo Changer and they r like hens teeth. There might be a universal Sony 1O disc that fits and lucky for you its only available in the states.
Tech the 6 disc alpine units only work with the SC range of headunits.