If you're trying to replace the windshield, the rearview mirror will need to be removed *before* the windshield is taken out. The method for removing the mirror depends on the year and model of your Grand Marquis. Generally, it involves one of these:
* Detaching a clip: There's often a retaining clip holding the mirror to the windshield. Carefully try to pry it loose with a small, flat, plastic tool to avoid scratching the glass.
* Removing adhesive: If there's no visible clip, the mirror might be glued to the windshield. In this case, you need to carefully work a thin plastic tool (like a putty knife or plastic trim removal tool) around the perimeter of the mirror to break the adhesive bond. Be extremely careful to avoid breaking the glass. Gentle heat (like a hair dryer on low setting) *might* help soften the adhesive, but be cautious as excessive heat can damage the mirror or the windshield.
Once the mirror is removed, you'll be able to access the windshield for replacement. The metal backing plate is part of the mirror assembly and shouldn't need to be separated. If you're trying to separate the metal plate, you are likely doing something that is not necessary for windshield replacement.
If you are attempting a windshield replacement yourself, consider seeking professional help. Windshield replacements are best done by experienced professionals who have the right tools and know-how to ensure a safe and properly installed windshield. Improper installation can compromise the structural integrity of your vehicle.
