If the mirror is detached, the problem is likely with the mounting mechanism, not the adhesive. You'll need to:
1. Investigate the cause of detachment: Carefully examine both the mirror housing and the car's door to identify broken clips, stripped screws, or other damage.
2. Repair or replace broken parts: You may need to replace broken clips or screws. Sometimes, you can find replacement parts from a junkyard or online auto parts retailer.
3. Re-attach the mirror using the correct method: Consult a repair manual specific to your Buick 98 Century year and model to find out the proper way to secure the mirror. This will likely involve re-inserting clips or tightening screws.
Using glue as a permanent solution will likely lead to a weak and unreliable repair that could fail and potentially damage the mirror further. If you're not comfortable with the repair yourself, take it to a qualified auto body shop or mechanic.