I believe the shoulder harness is not to exceed 20* from the shoulders to mounting point. Mounting them to the rear seat belts would definitely be more than that. In the unfortunate event of a wreck, and the car becomes an accordion (between you and mounting points of the harness) your upper body is no longer going to be held against the seat.
That happened to a Mini driver at Willow Springs a few years ago. He died in the crash.
I Googled but couldn't find any pix. Just lots of forums talking about it. I wouldn't run a harness without a rollbar/cage
1993 GT 310rwhp / 351rwtq
AFR FTI EDEL Combo. MM, T-56 3.73, Brembo, 275s on 03 Cobras all around