Okay,
First thing to know is that the stock radio head is Alpine; but that per Volvo's request they reversed the pins so that Alpine off-the-shelf stuff won't work with it.
The answer is called the "VolAlp" adapter. It basically reverses the pins back to the original orientation. (Just Google "VolAlp" and see what you get.)
You will need the VolAlp (which you have to solder in behind the stock radio), the CD changer, and the cable. - I got all three for under $200.00 Canadian, mostly from eBay.
Just make sure that you have the 4 digit reset code for the stock radio.
Installation is a no brainer - took me about 1.5 hours (wagon) to do it right.
Tell your friend it is really complicated, and guilt trip him/her for at least a two-four of premium beer.
Cheers!
Armie

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'94 850 154000kms '86 760GLE 272000kms