Mark,
There is not a lot behind the rear panel to attach the shoulder harnesses to. The rear panel is thin gauge aluminum and will not support a shoulder harness. There are anchors behind the seats for the lap belts. There is a frame behind the panel for the differential and there is a hoop that travels on the underside of the body above the differential frame. I am attaching photos of mine. In order to remove that rear panel you have to remove the seats and transmission tunnel, then the panel is removed by taking out some screws and carefully maneuvering it out. For the driver you can add a bar to the hoop above the bodywork for the shoulder harness, the passenger is the more difficult one.
By the way, you are not going to have that open space that you see above my differential housing, as that is where you gas tank is. I deleted the gas tank as my fuel cell will be in the trunk.
Jim
