Auto Dimming rear view mirror retro fit

Hi it's my first post here, so I apologise if i'm in the wrong area.

I wish to purchase an Auto dimming rear view mirror to fit my S3 (only came with a basic flip type mirror).
Has anyone done this? Can anyone confirm if it can be done please?

I can purchase the mirror assy easily on line from ebay for around ?90.

However, it'll need connecting up to somewhere and that's the bit I'm not sure about.

I don't have access to VAG COM yet either.