Or if you have a late model european car you can get the isobar system which is like a tether strap but is inbetween the back and the seat of the seat and is like a hook.
Tether straps shouldn't fall through a gap in the seats however they should sit either on a parcel tray or in the boot. I know the info I have with my car seat says it shouldn't be used in a small car or hatchback where the strap is likely to fall through the gap in the seats where it folds or sommething along those lines (sorry mental moment)