The mirror glass is held onto the motor by a short length wire "spring" - basically a straight piece of music wire pivoted at one end that hooks under a tab on the back of the mirror glass. There is also some double sided tape between the back of the glass and the motor.

The painted mirror housing has to come off first. To do that you have to flex the whole mirror housing forward and reach in a retract a stainless steel spring clip. It is a PITA. Then slip the cover off.

Just hope the anchor for the big spring doesn't break.