Practice removing the rear mirror glass in a junk yard. Move the mirror glass forward on one side as far as possible, so that you can get your fingers behind it. Reach behind the glass as close to the center as possible and slowly pull it straight out. Most mirrors are heated so unplug it before you rip off the wires.

To insert, just plug the wires in and push on the glass to set it in place.

The rear fog only works when the headlights are on, at least on my 1998. The 850s can have 2 fog lights if you break off the plastic plug on the passengerside. The bulbs are the same as the brake lights, so it is a good place to keep a spare anyway.

Have you tried cleaning off any corrosion in the bulb holder?

Klaus
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Always willing to listen, just not able to take direction.