1991 Camry. I havecracked the glass in the external mirror. How do I remove the glass and fix a new piece? Do I have to remove the whole assembly from the door?

 

The glass is typically glued to the backing plate, this may be some type of hot melt glue.

A mirror and glass shop may be able to cut one the right size. You would need to gently remove the old mirror and glue in the new. Might try heating the mirrow and glue.

Or get a complete replacement mirror at a junkyard or internet site. There is a cover on the inside of the car that prys off exposing 3 bolts that need to be removed.