I phoned GPC a week or so ago and they quoted me
Mirror Glass driv?


36.43
Mirror Glass pass?


27.03
including VAT. I think they worked out ?5-?10 cheaper than VW.
As for the 12v if you have leccy windas I would splice it into that, when would you ever use your windows and mirrors at the same time?

Then you could make use of the same fuse. If not just grab 12v from elsewhere and get an inline fuse holder.