Have you pulled off the interior door panels to have a look at the location of the door wiring harness? If not, once you do it might become apparent why there are no pictures with accompanying tutorials. You will be pretty much into doing this blind, if you can do it at all. Pretty hard to take pictures when you can't see anything. My thoughts are that once taking a look owners pretty much equate total pain = ain't happening. As an aside, a previous owner of my car decided to run some extra wires into the doors for some unknown purpose (they were just hanging loose) That owner ran the wires outside of the sheath.
Each door amp has 4 wires. Two signal wires from the head unit and two power wires. You might want to consider the option of repurposing the power supply wires for the signal if you are using a centrally located amp.
The high risk approach to pulling new wires into the door harness would be to cut the existing wires from the amp connector and use one of the existing wires to pull the new wire through the harness. You will have to lubricate the new wires to get them through the harness. Silicon spray might be an option; but, it tends to scrape off the wire too easily which means that you only get lubrication for the first couple of inches in the harness. Klein Tools and others make cable pulling lubricant which electricians use to pull cable through conduits. Home Depot might have it in reasonable sized quantities (20 l pail is a bit much for small jobs!). When electricians fish wire through a conduit, they usually wrap the new wire onto the fish wire and then wrap the connection with tape to secure it. You won't have room in the harness for a bulky connection like that. I suggest that you connect the new wire to the 'fish' wire with an un insulated butt crimp connector (less bulky than insulated connectors). This will reduce the risk that the wires separate in the harness half way through the pull which would leave your screwed.
If the existing wires in the harness have any twist in them, there is a chance that the wire pull will jam part way through. If that happens you may be able to pull back and them restart with more lube. If the wire fishing exercise does go completely south on you (the wire separates or gets completely knotted up in the harness), you will likely have to pull the door harness out of the car to fix it up. That would probably redefine 'total pain'; but, once that happens you should be able to do a sanitary job of pulling the new wires through!
Full disclosure, I have fished new wires into a short section of automobile harness; but, definitely not tried the NSX door harness. Having been into the door cavity to upgrade my window regulator my take is that this just has too high of a bloody knuckle quotient. Attempt at your own risk. However, if you are in there, do take the opportunity to upgrade your window regulator or at least clean and lubricate the cable and guides to minimize the probability of a future visit to the inside of the doors.
