i have just done it but they are not a straight fit. PVW got it wrong
the mirror base (the black bit that bolts to the door is a completely shape)
the way i got round this was to swap the lupo base with the golf base you can use your existing electric motor/wiring etc as they bolt up in the same place.
i have been collecting parts off ebay etc and the whole swap for both mirrors has cost me about 50ukp.
you can even use lupo manual mirrors as the casting in the mirror is the same for elec/man all you need the elec/heated/blue glass from a lupo/polo gti and you are away i also has the part no for the small driverside elec/heated/blue tint glass if you need it.
Just on with the Lupo mod at the moment pal , ive started taking pics as well, i got both sides from the scrappers for ?40 and an added bonus they dont need paintig, but they arent electric, but shouldnt be a problem i dont think. I will post pics soon as ive done, prob be nxt weekend. 
i have done this conversion.
can verify its a easy swap.
the only dificult part is removing the clip that holds the spring on the pivot of the lupo mirrors.
also the mirror glass isnt easy to get hold of. i no longer have part numbers but the stealers dont understand that you want a lhd electric mirror in a 130mm not 150mm!
and the glass is about ?25 each for electric heated ones.?
but worth it!
i've done the drivers side, and converted a manual lupo to electric, and am on the hunt for a passenger side one. The small mirror on the passenger side actually comes fitted to loads of cars, polo lupo arosa ibiza and something else i can't remember!
its a fairly easy job, get a complete mirror from a breakers though, but finding one is hard! don't buy mine either!!
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