On my 1999 S70:

First remove the wipers (just pry off the nut covers with a small screwdriver and then unbolt),

Then remove the black plastic cover under them. There are six (?) torx head bolts and you also need to undo the two drain hoses on the bottom of the black plastic cover. Pull to cover out. If you have a cabin filter, now would be a great time to change it (located on passenger side under black plastic cover).

Then you can see the four 10mm-head-bolts that hold the dash to the firewall (one of mine also attaches part of the wiper assembly). I don't know what these screw into, but I would not torq them down too tightly for fear of breaking something. I found it helped to have a helper push the dash against the firewall from inside the car.

Hope this helps. It stopped most rattling on mine.

Shane
--
1999 S70 non-turbo