With most headrest packages, they are designed to have all the wires required to hook up the headrests run down the posts and through the seats to the vehicle floor.  This is very convenient to help hide the wires from view and also maintain that factory look.

After the wires are near the bottom of the seats, you can choose to hide the wires under the seats, under the floor mats, under the carpet or under the center council.

Please note, some vehicle seats have a removable back and others do not.  The difficulty level to get the wires down through the seats to the bottom will vary from vehicle to vehicle.  Most seats that don't open on the back will have a way to open on the bottom providing better access to the cables.