Volvo dealers have brand new radio heads with single CD player builtin and will still hook up to your trunk mounted player. Just specify you want black instead of grey. It comes with a code card. You do need to give them your old radio, of course. Mine cost me about $320, after taxes, no shipping.

Klaus
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98 V70Rawd(101Kmi), 95 854T(85K mi), 75 164E(173K mi)

I did a quick google search for 850 radio's with CD changer and found a factory refirb from a salvage yard in Atlanta - volvoparts.com Phone 404-352-3402. They sent it with warranty for around $275 + 978 shipping. The cost of an aftermarket chager was around that and the one banger works just fine and slid in the opening in 10 mins.
Nedly '95 850 91k

You can get an Alpine unit that will fit and will work, but you will need
to change out the cabling running to your changer.
You'd be better off e-baying a replacment Volvo head unit SC810/812/etc. and
saving yourself a lot of grief. Just be sure to get the 4 digit re-set code
for the radio from the seller.
Armie
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'94 850 154000kms '86 760GLE 272000kms