Because its location is not consistently described in a single easily accessible way, the best approach is to:
1. Consult a repair manual: A Chilton's or Haynes repair manual specific to your 1995 Mercury Mystique will have a diagram showing the exact location of the PCV valve.
2. Search online for a diagram: Search "1995 Mercury Mystique 2.0L PCV valve location" (or whatever your engine size is) on Google Images. You should find some diagrams that show its placement.
3. Look closely at the valve cover: With the engine cool, carefully examine the valve cover. The PCV valve will likely have a hose connected and might be easily visible.
Remember to always disconnect the negative battery terminal before performing any work on the vehicle's electrical system.