I don't think the '93 has one. I replaced my stock cassette player with a nice Pioneer CD head unit last year, very easy to do. Once you remove the stock radio, which is easy, just pop out the side tabs and pull it out, then unplug the antenna lead and the harness. The opening is standard double DIN size, so any aftermarket radio using DIN size will fit it. My head unit was single DIN, so I got one of those nice little plastic single DIN sized cubby boxes to go under the stereo. Wiring is a snap, most good stereo shops sell the adapter harness to fit the 850, so you don't have to hack into any factory wiring. Just plug in the aftermarket harness to the factory harness, connect the stereo wiring to the aftermarket wiring using crimp connectors or something else suitable and your ready to go.

Either that or you can find a factory Volvo CD stereo, rebuilt or used, off somewhere like Ebay. I found lots of them when I considered going that route, but they aren't cheap. In fact, my Pioneer was on sale and was cheaper that a Volvo unit, and I'm sure sounds much better.