The requirement to manipulate this vehicle is to have the ability to transfer mana through some form of contact. This contact is not limited to merely human touch and can be accessed through a medium. When Leo had first proposed his idea, he had demonstrated this technique after using the Low-level spell [Earth Creation - Metal] to create a long, thin strand of metal extended from his finger, which he then connected to the carrier's underside. He had actually drawn inspiration for this idea from his very first assignment in that Goblin infested ruin.
Of course, such a thing alone wouldn't provide the best handling of the vehicle's swift mobility. Thus, unbeknownst to those who weren't familiar with Mercury's blueprint, the metallic wire Leo had connected was attached to a specific area that led straight to the carrier's magic core. This granted superior access to its functions, enabling him to control it from a farther distance so long as the wire doesn't get broken off.