Rear View Mirror Issue

I have a 2004 325. At nite the rear view mirrow has a portion that stays extremely dark, while around it it is normal.

Any one have this problem?

Can the mirror glass be replaced? How? Can it pop out or does the entire assembly need to be replaced. Probably pricey as the assembly houses the alarm flasher and the garage door opener.

If it is replaceable, any good web sites for the OEM mirror?