Go to the hardware store and get some long stainles steel oval head machine screws that fit in the mirror holes and some wing nuts. Slightly enlarge the mirror mount holes in your door so the screws slide in. Remove the inner door panel, then with the window rolled to the right level you should be able to hold the wing nuts inside the door and tighten down the screws at the mirror. This is what I did, worked great.
you may want to loctite the screws if possible.. this will prevent the nuts from coming loose due to vibration. If you don't want to use loctite, try to find some locking wingnuts (they'll have some nylon next to the threads that prevents the nut from rattling loose.. they're sometimes called "nylock nuts")