MMM as far as I can tell it looks like it's for the touch screen as it plugs into p13 and is the same plug type as p10, p12 and p17 and a couple others. Could be wrong though :)
Just a note, I FINALLY managed to unhinge the 3DS found a youtube video with a comment that helped lead me in the right direction. It's a huge pain to get off but makes threading the ribbons easier and lowers chance of dmg.
To remove the hinge go to t he hinge that does NOT have the ribbons. Beside there is a piece of plastic that sits on top of the LED power light, look for a small opening, if you look carefully you should see white plastic. Make sure to close the 3ds carefully and push the piece of plastic towards towards the middle of the case (this can require a lot of force). The hinge should now detach be careful. Once the white plastic is far enough in, the plastic piece surrounding the LED plastic should come up quite easily.
Now you can thread the ribbon cables on the other hinge through quite easily.
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