this website might help with install of a cd changer. http://au.geocities.com/ozbrick850/cdchangerinstall.html
it has a pretty good description of what you're looking at in the back of the head unit and which wires are for what. i installed an alpine changer, the volalp connector is what you need to connect an alpine to the volvo head unit, available at crutchfield for maybe $15. i would assume that the volvo changer would be plug and play. i mounted my changer in the third seat area. i never use it(the seat) so popped the bottom seat off and mounted the changer where the seat was. plenty of room for cds under there too. and don't forget to have your 4 digit code handy when you disconnect and reconnect the radio.