That shouldn't be too difficult. Remove the door interior trim panel and pass a wire through the rubber conduit from door to main body. That'll get the wire inside the cabin, behind the footwell trim panel. A little lubrication may be needed to feed it through. Use a fairly stiff wire, it's easier.
The other end just goes up the inside of the door and into the mirror, following the existing wiring. You might need to remove the trim panel from the corner of the door, or even remove the mirror to get the wire in there.
It's a custom job though, so DIY or see a custom shop, I wouldn't imagine your MB guys would be much help on this. Also, modification of the electrics voids the warranty in most cases.
We have done this for a customer before, its no big deal unless you are the one paying for it, those mirrors don't come cheap.....
IIRC this guy wasn't bothered about changing the wings, so we left the repeaters in and just spliced into their wiring for the feed to the mirror side repeaters, so when he indicated all the indicators lit, including the ones in the wings.
Just run the wiring from the wing into the door and connect it up, no biggy
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