you could either strip the back seats out for a better working position, or you could feed them through the gap.
They mount perfectly on the cross-frame just where the back support is. There are already existing holes and many to choose from on where to bolt the harness belts in. A couple of bolts and a bit of hole enlarging and your set.
Just make sure you don't mount them on an acute angle to the seat as it is a defect and will probably end up saving your life.