Cars that have an integrated head rest (BMW Z3, Porsche 911, etc) pose a problem for proper installation of shoulder belts for an aftermarket harness. If you are very tall and broad shouldered, these seats may not give you any issues. But if you are short enough so that the seat interferes in ANY way with the path of the shoulder belts, you've run into a problem.
The solution (without swapping seats) is to modify the seat so that there is a place for the shoulder belts to pass through. These grommets will give you exactly what you need to do a good job. They will help finish off the upholstery work with a professional look.
We must stress that You are the one deciding to cut open your seats, not us. There are many solutions for proper harness installation - this is only one of them. These are sold as one mating pair (front & back). If you need to do two holes, as if you were putting in shoulder belt pass-thru's you will need to purchase two mating pairs.