Well I did it when I had cloth. But I would do the following:
Remove the headrest of course.
Get a hanger on some type of metal rod.
Wrap the ends with tape and then feed one end thru the headrest post and guide it down thru the back of the seat until it comes out of the bottom.
Then with the end still exposed thru the top of the seat attach the wire to the hanger/rod and tape them up well (you may need to stager the wires to ensure they fit thru the opening. Then pull the rod/hanger from the end exposed at the bottom of the seat.
Congrats! you have wired your seats for headrest monitors.