Do it yourself, remove the rear door panel on your Mercury Grand Marquis, made between the years 1998 and 2002. You will need flat blade and Phillips screwdrivers for this procedure. The removal process is not that difficult and you should be able to do it in less that 20 minutes.
In the beginning, open the rear door and start by prying up the window switch. You can do it with the tip of a flat blade screwdriver, inserted as seen in the image below. Lift up the plastic carefully and then use a Phillips screwdriver to remove the harness beneath it (the plug is held by two screws).
Once you remove the window switch, you can move further and pry out some plastic covers that hide the Phillips screws securing the door panel. You can start with the cover behind the door handle. Using the tip of the flat blade screwdriver, remove it and then go to the two small caps at the door pull, as you can see in the picture below.
Once you deal with them, you will be able to remove the Phillips screws. They are located as you can see in the picture below.
After you do it, you can simply lift the panel up and remove it. It has some plastic hooks that hold it attached to the door frame. Make the repairs needed and install it back reversing the above procedure. This is all you have to do, if you need to remove and reinstall the rear door panel on your Mercury Grand Marquis, the model manufactured between the years 1998 and 2002.