just noticed the other day that my passenger side mirror does not defog anymore; the driver side is fine.
anyone know what broke? or maybe the part number also?

Most likely the heating element on the back of the glass is broken. Just replace the glass. There is a slight chance the wire connectors may just have come loose and become disconnected. Use the link to find the part number. www.realoemparts.com

ah so the heater is on the mirror glass itself then. will have a look thanks.
it is possible to open up the mirror cover without taking the mirror off right?

I had not a need to remove the mirror off of a E60 yet. But I did need to replace the mirror on my '95 530 because passenger mirror did not defrost. On the 95 E34 the mirrors can be removed from the housing as it is attached to the motor with 4 plastic clips on back of the mirror. I just had to position the mirror as high as it will go and ply it off. NOT SURE IF THIS IS THE CASE FOR THE E60. Maybe someone with experience will chime in.

yeah think i'll get one of my mechanic friends to look at it first before i break something :)