General rule of thumb when it comes to this;
- Don't use a 5pnt unless you have seats designed for it.
- Don't use 5pnt if you don't have a rollbar.
- Don't use 5pnt unless you HAVE A ROLLBAR!
It's a matter of safty issues. If you want to see some bad pictures of people who have used a harness without having a rollbar or the correct seat I will be more than glad to show ya (however I'd prefer not to).
Using a 5pnt straps you into your seat. So if you ever roll a car, by by snapped neck, by by your life. Having a rollbar permits the roof to cave in on ya if your strapped into your seat.