The 1999 Ford Expedition uses a spring grip on the rear view mirror mounting tab that is slightly difficult to remove. It requires a special tool to compress the spring properly without breaking the windshield. The tab attached to the windshield is held on with special rear-view-mirror adhesive. This is the only glue that will work for this application. Generally, the only reason for replacing the rear view mirror is due to the mirror falling off due to a failure of the adhesive, or failure of an integrated electrical component.
Remove the mirror using the special tool to release the mirror from the tab attached to the windshield. The tool is shaped like a T with the top of the tool having a very thin blade and the centre being the handle. Insert the top blade between the top spring clip on the mirror mount and the tab.
Pull down on the tool while lifting upwards on the mirror. When the spring clip releases the mirror will slide upwards off the tab on the windshield. Do not attempt to remove the windshield tab also. It is to be reused because any attempt to remove the tab will result in a broken windshield.
Install a new mirror by sliding the mirror mount down over the windshield tab until it locks in place.
Insert the special tool between the mirror mount and the windshield tab. While pulling down on the tool to release the spring clip, lift up on the mirror to slide it off the tab. Mark the side of the windshield tab that faces the mirror mount. This side will have a small indentation in the centre of it and no glue. The point here is that you do not want to install the tab on the windshield backwards. The mirror will only mount on one side of the tab and once the tab touches the windshield it will not come off to reposition. This mistake requires a new windshield and it is easy to do. Another helpful tip is make sure the side that is completely smooth is the side that gets glued to the windshield.
Clean the glue off the windshield side of the tab using the razor blade. Clean the windshield side of the tab with alcohol. Mark the position of the rear view mirror tab on the outside of the window. This will aid in locating the mirror correctly as it is installed. Once you've the inside of the windshield, there is no way to see the actual location.
Scrape the glue off the window using the razor blade. Clean the area thoroughly with alcohol. Open the rear-view-mirror adhesive kit and take out the glass tube of primer. Bend the tube to break it and release the primer inside the tube.
Remove the cardboard cover from the tube to expose the brush. Hold the brush down so the primer will soak the brush. Apply the primer with the brush to the area the windshield tab is to be located. Allow it to dry for five minutes.
Open the tube of glue and place one drop dead centre on the windshield side of the tab. Immediately press the tab glue side to the windshield, making sure the tapered end or narrow end is facing up. Hold some pressure on the tab for one minute. Release the tab and allow a half hour for the glue to set properly. Install the mirror by simply sliding the mount down over the windshield tab until it clicks.