Unless you insist on a stock-looking interior (not a bad thing, as who would break in to steal an old Volvo head unit?) you're farther ahead to buy something modern and replace the original. I bought a modern unit from Crutchfield for my 89 745, and they supplied a wiring adapter along with it. Removal of the Volvo unit was fussy, but do-able, and the harness adapter made it truly plug-and-play. It bypasses the separate Volvo amplifier and connects the new head unit to the speaker harness.

Sound is at least as good as the original, and FM radio reception is better. As another poster mentioned, you can connect your iPod, phone or other music player. Some units have Bluetooth capability.

It's tough to find a modern unit that doesn't have a dazzling, video-game display. First of all, I'm way too old to be impressed by that goofiness, and it is totally out of place in the blocky, old, black Volvo dash. You can try visiting a place like Best Buy or Fry's, etc., to view units in action before you buy.

Re: speakers...my inspection of those OE units says they are a rather cheap setup, with paper cones and tiny magnets. After 20-plus years they probably have deteriorated. If not, great, but listen closely for any distortion, buzzing and so on. New speakers can make an absolutely huge difference in sound quality.
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Bob: Son's XC70, daughter's 940, my 83 240, 89 745 (V8) and S90. Also '77 MGB and some old motorcycles