To find it, you'll need to:
1. Consult a Repair Manual: A Haynes or Chilton repair manual specific to your 1998 Mercury Mystique and its engine code (found on a sticker usually located under the hood or on the driver's side dashboard) will be the most reliable source. The manual will have diagrams showing the location of all engine components.
2. Use an Online Parts Diagram: Websites like AutoZone, Advance Auto Parts, or RockAuto allow you to select your vehicle's year, make, model, and engine. Their parts diagrams often show the solenoid's location. However, the diagrams may not be as detailed as a repair manual.
3. Visual Inspection (use caution): Once you've consulted the resources above, carefully inspect the engine bay. The VVT solenoid is usually mounted on or near the cylinder head, often close to the camshaft or valve cover. It's usually a small, cylindrical component with electrical connectors. Be cautious when working around hot engine components. Let the engine cool completely before attempting any inspection.
Without knowing your specific engine code, I can't give a precise location. Finding the repair manual or using an online parts diagram with your engine code is essential for accurate identification.