This would probably be better in Technical, but you have to remove the glass.
There are two ways to do this depending on the year of your model...yours seems to be the later if it's still the broomy 1999.
On instructions from Joe at FCSS I made a noose which I slipped under the mirror glass (first having removed mirror from car!), and left 3 longish "legs" of cord attached to the noose which I knotted together at the ends like a sort of tripod.
Then the brave bit is to hang the knot over something substantial like a chair back and then suddenly pull down firmly on the whole housing in the opposite direction to the knot so as to spring the glass away from it.
The glass in these models is simply held on by circular plastic jaws, not the horseshoe shape spring of the older type. It might be an idea to spray some WD40 in behind first of all as I guess all the action will be a surprise after 17 years
If the noose is nicely positioned around and behind the centre of the glass then with luck you won't break it - to my relief mine worked first time.
Once in, remove the electrical attachment then the motor, after which the remaining screws are fairly obvious - except the last which holds the centre black plastic bit in. Again you may need a skoosh of penetrant fluid.
Good luck