good job on the relo ygmn, now I just spotted this topic and maybe I can add some insight, having done my headrests and run the wires.
do not remove the seat cover unless absolutely nec and you should always avoid it if possible (so hard to get things "right" again). With that said, I think the Viz system actually runs their wires right through the metal post of the headrest, which I did not do - I have a single cable that runs right from my headrest (alongside the post) into the seat (alongside the post). Not at all unsightly, and my headrests are always in the "down" position so it is never even seen anyway. I had to pop off the plastic post guide (at this point the headrests are off) and cut the fabric slightly to enlarge the hole to allow a little more manueverability. (I have leather seats with cloth inserts). Then, I took the other side of the wire harness (the M&D units I used have a short run from the monitor that mates with a plug into the longer cable run) and taped the end plug to a coat hanger wire, and fed that up the back of the seat from the bottom, near the c-console side - there is a natural entry point "pocket" at the bottom of the back, on the right side - a 3" sleeve where the leather "panel" that frames the main panel terminates.
now if you have a wire that is run inside a post, I would take a long coat hanger wire and explore what you can do inside the post hole - I suspect it may be open ended at the bottom and you could direct the hanger to some exit location on the seat back bottom, perhaps the same one I used.