The internet is silent on how to rebuild your headrest, so I decided to tear into mine and show the readers how to do it. The headrest is released by pulling it up then pressing a button protruding from back left side of the seat. Once the headrest is out you open up the zipper by pulling each tab (I used one of the office clips). You will notice that the zipper is held together by clips - there is no slider.
Once the holding clips are out, you can open the zipper out, unbolt two headrest bars then pull the headrest assembly glued to disintegrating "horse hair." Clean the insides then carefully separate the horse hair from the foam. The foam will be used as template later.
Using hi-strength adhesive foam attach 1.5 inch foam around the headrest assembly. I had some 1/4 inch foam with backing laying around so I planned to use it to reinforce the headrest. You will have to trim the sides front and back of the headrest is relatively thin and only the top is thickly padded. Based on the thickness of your reinforcing foam you will have to figure out how much to trim. Use serrated knife to trim the foam then glue "ears" on the top.
When finished, force the padded headrest into the vinyl then take your time lining up the edges and "ears." Then insert the bars and when the pouch is still unzipped, screw them in. Run the zipper slider through the zipper and secure it with metal ends. Tuck those in and your headrest is finished!