i had a massive argument with seat uk over this very issue - i ordered my 05 plate car when the dimming mirror was standard fit, the car arrived, built as an 05 model with fixed rear seats, but with a bog standard mirror - dealer said it should beon and told me they'd fit one, then dealer goes bust, seat uk pick upthe case, and get another dealer to order in the parts for the job - they take the car for an investigative look, and decide it needs a new loom in order to plumb in a mirror - dealer B is keen to do the work, as they quote 2 days labour in stripping down the car including a dash out to replace the front half of the car loom....seat uk arn't happy so get on to me telling me that they told all uk dealers of the spec change during my build window, and that i shouldn't have the dimming mirror, hence seats refusal to pay the dealer 2 days labour to fit one.
if there is a loom already in place on your car for the mirror, then great - go for it, i'm not sure how clever these mirrors are - if they simply need an earth, 12v+ and perhaps a feed from the light switch, or if they are can-bus wired and react to the signal from the light sensor to tell then when its dark rather than the light switch position.
pleas let us know how you get on. - i think i'll have a look in the headlingin of mine to see what scared seat uk off fitting one to my car