I've seen a few pictures on here with the upper parts of the harness, that go over the shoulders, attached to the floor (probably using the rear seatbelt anchorages.) But I always thought that was supposed to be dangerous - in an accident wouldn't it pull down, potentially damaging the spine?
I take it the harness bar(s) is/are above shoulder height?
Probably not what you're asking. You probably want to know how to physically attach the ends to the cage.