I put the shoulder harness portion on the floor behind the front seats. I just tucked them under the seat if I had rear passengers. As I mentioned earlier, I'm sure the anchors for the tail straps will stick up out of the rear seats a bit.
The quick fits would be much nicer. But if you hardly ever carry rear passengers or just put a car seat in the back, then the Ralleye's will work.
For adults and grown children, I would imagine that it would become a minor nuisance for most.
2015 F34 328iX GT Glacier Silver
2011 E90 M3 Monte Carlo Blue
2004 E46 M3 Imola Red