Are you installing completely new headrests with built-in monitors or just monitors into your existing headrest? Here are some pics that can hopefully guide you some. I installed monitors into my existing headrests.
Not sure how far you've gotten so far, so I'm posting whatever Ithink may help.
Once I installed the monitors into my headrests, I drilles a hole to get the connector and cable through.
The pin hole in the picture below is the release for the headrests. If you stick a small pin or tool through there, that will release the headrests so you can pull them completely out. I put a small slit on the outside of the adjustment lock and ran the cable through there.
If you look underneath the seatback, you'll see where you can unsnap the white plastic trim to pull the upholstery up so you can reach up into the back of the seat to pull the cable through.
Once through, be sure to route the cable and secure it away from the seat rails, etc so they don't get pinched while the seat is moving. Here is a final pic: