Is this for a street car? If it is then as stated you can't legally have a back seat, or it can't be used, and why would you bother for a street car? I have harnesses in my 180B SSS, but I'm reverting back to the rectractable seat belts for street, as it is just a pain getting in and out, and putting them on.
If it is that much of a problem, go looking for a 200B SSS, where they have a locating arm which mounts near the roof, and drops the belt down to shoulder height. There is a mounting point under the roof lining for a belt if you want to go that way. Obviously you'll need to use a retractable belts.
As safe as harnesses are, they really are made for race vehicles, not street cars. They draw too much attention from the Police, and in my eyes are a hassle to use. My 2 cents.