I have a 2005 MO so I can not speak for the 2012, but I did replace my mirrors with MUTH signal mirrors so I know what is generally involved.
Taking off the interior door panels to access the mirror mounting bolts/screws is a fairly straight forward process. A few screws and a dozen or so plastic fasteners gets you to the interior of the door. Mounting the new mirror glass is simple, too. Even running the wire for the heating element is relatively simple. The fun part is finding the correct wire to tap into. Nothing is labeled. You need to find the correct color code on the wire and splice into it.
If you are comfortable with tearing into a new car then you should be able to do each mirror in about two hours or less. If you prefer to let someone else get their hands dirty I suggest finding a local body shop or custom shop and get a quote from them.
I would avoid your Nissan dealer because I would bet they have never retrofitted a heated mirror to an SV MO.