A lot of people (including myself) have installed their harnesses but bracing the rear at some points under the rear seat.

Good for holding you in place but...

One problem with this is in an accident (knock on wood), your body shifts foward, most the weight is caught by the top 2 straps. Since the attachment points are below your shoulders, it will actually have the effect of compressing your body down into the seat, which can hurt your back. Better than going through the windshield ofcourse...

The proper way to attach harness is to a roll cage which is usually at a higher point so in case of impact, the harnesses hold you in position rather than make you shorter.

Problem is most of us do not have rollcages ... which is why its a good investment if you are into autoX or any kind of legitamite racing. Added plus is it stiffens up the body (specially for us who have sunroofs). Plus using harness on the street is foolish, as there are no blind corners to look around on a track, you will need the give of a standard belt to do that.

Also when in use make sure the lap belt is tight enough so you don't slip out the bottom ... not good for the knees.

 

Mag, you can fix the rear straps in the same location as the stock rear belts. I have mine hooked there and still have use of the stock belts. But you are right in what you say.
problem with roll bar/ cage/harness bar- you really should NOT let anyone use the rear seats.

 

Yea thats where I have mine. True about the back seat passengers but there are different configerations out there, some more user friendly. Cusco roll cages are not bad, they more or less follow the frame of the car. You can always take off the cross bars when not on the track... and of course wrap the cage with the foam material like they had on BMX bikes cross bar to protect our nutz.

Sparco has a new belt harness bar out but that looks pretty neat but a head splitter for anyone sittin in rear.