My '93 had it mounted right under passenger side rear deck speaker in the trunk. But it started messing up and I got a replacement and moved it a little farther away from speaker and it's magnet incase that might have cases my problem. I've seen them mounted in the trunk under both rear deck speakers, bolted to the trunk floor and even side ways on side of trunk. Newer, smaller ones go in the glove box.

So moral of post... put it anywhere you would like to! No pre-drilled holes were made by volvo for the 93s all I've seen where dealer installs.
I would suggest running wires first to be sure you got enough and if putting in trunk I like attaching to rear deck to keep floor of trunk free. Just be sure trunk lid supports won't hit unit when you close the trunk.

I don't have a picture but look on ebay at '93 850 trunks with CD players and you'll probably see several.