Yes, when using the car seat rear-facing, you must always install the seat with the support leg. That’s how our car seats are tested and approved. Before installing the support leg you need to make sure that there is no storage compartment underneath the car floor. If this is the case, the lid needs to be removed and the support leg installed in that space and not on top of the storage compartment. Most car manufactures offer a storage compartment filler that also be used but please consult with your car manufacture or dealer.