The 850 and the V70 (1998-2000) have hydraulic shocks that help to open and hold the tail gate up and in place once opened. With time the tailgate struts will fail internally and you will notice that the tail gate will not stay up. More than likely only one of the struts has failed however if one has failed the other one is more than likely not far behind. This post will help you remove the trim thats in the way so you can get at the struts and replace them.
Start by raising the tail gate, you might want to have someone helping to hold the tailgate open, or use a broom stick or something to hold the tail gate up while you are working. When you remove one of the struts the tail gate WILL NOT stay up. Start by removing the speaker grill, this will enable you to get a better grip on the plastic trim to snap it out of the holders or clips that are holding it in place.
There is one 25torx screw by the coat hook that you will need to remove before you will be able to snap the trim loose up to the back seat pillar.
Once the speaker grill is removed and the 25torx screw, you will be able to get a better grip on the plastic trim to un-snap it from the clips that hold it in place. Gently pull the trim inboard and then down to free it where it joins the center plastic piece thats on the back center of the roof. You might want to wear light gloves, the plastic can have sharp edges on it.
Once you have the back piece loose gently pull down and the trim will come out of the yellow clips that hold it in place. You can let the trim hang there or take it down all the way.
Once the trim is out of the way you can use a screw driver or a small pry bar to pop the strut from the ball that holds it in place. The strut is held in place like a ball and socket. The strut is the socket and it has a "C" clip in it, to hold it in place on the ball once snapped into place. Do one side at a time, and you might have to raise and lower the tailgate just a bit to get the strut to line up so you can pop it onto the ball. I usually snap the front part of the strut on first cause it's up under the headliner just a bit. By doing the front first you have more room to manover the tailgate to line the rear up and snap it into place. Should you forget which end of the strut goes forward, just look at the other side thats still in place, or look at the above picture.