Re: 2000 Mercury Mystique door panels...
In general, most of the panel pops off or out so to speak. Birst you need to remove several screws first. They are usually located, at the arm rest(1 or 2 screws), inside pull handle, (the one you pull to unlatch the door) sometimes there is an even bigger screw behind this one also.
Some of the fords, the panels pull up to remove instead of out,but most pull out. They have clips on the back side of the panel that snap into the door skin.
some cars have a trim in the corner, right above the panel that need to come off also.