My wife's toyota has an auto dimming mirror, and it's a feature I really like.

Saw this one on fleabay:-

http://cgi.ebay.co.uk/AUTO-DIM-DIMMING-REAR-VIEW-MIRROR-VW-AUDI-SEAT-SKODA-/230343226775?pt=UK_CarsParts_Vehicles_CarParts_SM

Anyone tried fitting one?

There was a guide on here a week or so ago. Looks more trouble than it's worth IMHO, and it's more expensive to retrofit too.

Most of the mirrors are mounted from the roof, not sure, but you may have to wire up a switched live so it works. Might have a go myself if i ever find one in a scrappys.

Here's a easy one to fit My link

Getting a switched live up the A pillar and into the headlining shouldn't be too difficult. Could even tee off the sunroof switch as that has to have switched live there somewhere?

Should have, if you don't have one get a multimeter they make it alot easier. I think most manufacturs use the same sized ball fitting, so i'm wondering if this is any good My link