The reason you cannot just install 5pt harnesses blindly without a cage has to do with the angles of the rear belts. If you were to wreck, they can cause spinal compression and really fuck you up. They do make special harnesses that mount far back near the rear speakers on most cars that will allow you to run them without a cage safely.

WIth that said, I would go with Simpson Camlock harnesses. Do *not* get non-camlock if you drive the car frequently, they are a PITA.