I just completed a subwoofer install in my '13 this past weekend. It went very well.
Power: Easy access through the firewall behind the glove compartment. There's a tube for the rear washers through the firewall, and one inch from it is an unused 1/2" hole that's plugged with a rubber grommet. Pop it out, cut a slit, and route the heavy gauge power feed from the battery. I used 6ga to the amp located in the trunk space.
Speaker leads: I'm afraid you'll have to use speaker level inputs to the amp. For a sub, these work just fine since THD (Total Harmonic Distortion) isn't a real issue for bass input signal.
If required, get a Scoche line level converter to reduce signal to RCA Line.
I found the speaker wires colors here: http://www.mynissanleaf.com/viewtopic.php?f=36&t=3308
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Front Left (+) Blue / Grey
Front Left (-) Violet
Front Right (+) Red / Grey
Front Right (-) Brown / Grey
Rear Left (+) Violet / Grey
Rear Left (-) Light Green / Grey
Rear Right (+) Light Green / Grey
Rear Right (-) Pink / Grey
Access to the speaker wires is also easy: pop the upright trim piece behind the front seats. Speaker wires are the only twisted pairs in the loom, colors match, and easily accessed where they enter the door segment. For remote turn on, I just used an accessory power lead from the fuse panel (Driver dash left).
Found my super black 2013 Leaf.