If the 850 is bolted together like a V70, then try this on a warm day (when the plastic is a bit more flexible):

1. Take off the wiper blades and unbolt several screws that hold down a black panel at the base of your windshield.
2. Tighten 4 or 5 bolt heads that go through the firewall. Have a friend push forward on the dashboard while you tighten the bolts. BE GENTLE! I do not know what kind of connector is molded into the dashboard, but you do not want to shear or strip it.

If you have access to the intake for the cabin air system (the right side of the car at the base of the windshield):

3. Clean all the nasty sludge, leaves, and mold you will find down in there. Use a strong cleaner like Simple Green or 409 to scrub everything you can reach. On the V70, you can reach the fan blades and clean them one by one.
4. Spray Lysol or another antibacterial agent down the tube while the AC is running at high speed.
5. Reassemble your car.

I tightened the dash on my V70, and I hope these instructions are suitable for your 850.