you need to feed wire down through the back of the seat - you get to it with the same path a headrest monitor would take. basically a hole and grommet next to the headrest posts. it will come out at the bottom of the seat cover, then you will want to zip tie the speaker wire to OEM wire and mounts to keep it free of the track. lastly, zip tie to OEM wire harnesses that go to the floor, then hide under carpet (tying to OEM wire when possible).
you don't want to hear the rear wave in the front seat, so a small enclosure is recommended. you could use PVC caps that are wrapped in deadener then use an adapter to flange for a flush appearance. lastly, be sure to high-pass the speaker based on the enclosure/response combination (probably greater than 300Hz)