It is critical that the bonding surfaces be absolutely clean!
Here is part of the instructions from the 98 GM GP Service Manual 2nd Ed.
3. Use GM Glass Cleaner G/M P/N 1050427 or polishing compound on a paper towel on the inside of the windshield surface in order to clean..... Rub until the area is clean and dry. When dry clean the area with isopropyl alcohol saturated paper towel in order to remove any traces of the scouring powder or cleaning solution.
Important: If the original factory installed rearview mirror is to be reused, all traces of factory installed adhesive must be removed prior to reinstallation.
4. With a piece of fine grit (320 or 360) emery cloth or sandpaper, sand the bonding surface of the new rearview mirror support or the original factory mirror support.
5. Wipe the sanded rearview mirror support with a clean paper towel saturated with isopropyl alcohol. Allow to dry.
6. Follow the directions in the manufacturers kit.......
As for the removing the little rear view mirror support (the small piece that attaches to the window) from the rearview mirror.
2. Use a small flat-bladded tool in order to remove the rear view mirror from the rearview mirror support.
Hope this helps,
Joe