I dont run a harness for that reason as much as I would love to.
no cage-no harness.
worst case if you go over in a car with no cage and a harness plenty of things car happen.
if its not a fixed back seat the seat may break. and if not it will hold you upright in the
seat and the roof will come down and squash you bad.
saying that i now have a cage in the silvia but no real way of mounting it correctly so i just run the stock
seatbelt.