As with _any_ sanctioned motorsports. Check the rules. Som clubs will allow the harnesses to be mounted directly to the harness bar. Others will require that they be bolted to the firewall. In either event, if you're not using an FIA approved seat, especially if all you're using is a stock seat, you REALLY should be using a harness bar to guide the harnesses.
Joe is right about compression injuries. If the harness is
pulling down on your shoulders, you run the risk of spinal compression in the event of a severe impact. The shoudler straps of the harness should pivot from a point betwen 5 degrees below to 30 degrees above the driver's (and/or passenger's) shoulders. This means that if the seat itself is not providing that pivot point (and it shouldn't anyway), that the harness should be mounted at some point either directly behind the shoulders or preferably slightly above.
For us vertically challenged drivers, getting the harness pivot point above the shouders is usually not too difficult. But those of taller stature, you need to be more carefull of where those harness are A) mounted to, and
where they pivot from.
-Josh2