Don't know about any volvo units but you can easily install a standard single din CD player in that space. I put one in my son's car a couple months ago. Got a wiring harness from Crutchfield ($12) and a pocket to fill the rest of the hole from Best Buy ($15). Crutchfield might tell you it won't work (that's what they told me) but many others on this board have done it as well. One hour start to finish if you are slow like I am, that includes soldering the wiring harness and filling out the warranty card.

Note the Volvo speakers are 8 ohm, all new stereo equipment expects 4 ohm speakers. It doesn't seem to matter but you might as well be aware.

Another option is putting a changer in the glove box.