I had this problem with my Alfa Romeo 166 !

I thought this was a typical italian car mishap, but it seems beemers can have the same disease :o

my alfa dealer told me this was caused by one (or more) anti frost wires in the rear window that malfunction

best way to solve this, is to change the rear window

you can also try to find the defect wire and to connet it with the next wire, but this is not very simple to do i guess

I never got my Alfa fixed, just sold it :D