note...not all 850's had the wiring in.....you MAY need to put the cable fro m the head unit to the boot in...

do you have a volvo or alpine cd unit, the plugs are different, if its alpine then you will also need a cable and a volalp converter unit to control the cd...

volvo mounted the cd unit vertically in the boot on the left hand side into the 'cheek pouch' along side the spare wheel well, if you have an alpins cahnger the alpine mounting gear can be used if your carefull and a little inventive to 'sandwich' the boot floor material after you cut a hole to drop the changer thru (the material is like a carpet with a thick rigid plastic backing that you can cut with a craft knife)

i did the cable, volalp, cd mounting testing etc in an afternoon without hurrying...you removethe radio the passengerside under dash panel (you need torx heads to do this!) the trim panels along the door cill area and loosen most of the interior panels in that side of the boot area (again torx!) and then run the cable install the changer test then button it all back up again. it sounds hideous, but if you take some care and dont rush its actually easy!...

if you want any pics let me know and i can take some...