An assembly for implanting a medical device such as an electrode array. The assembly includes a sheath that is disposed over the medical device and a core that supports the device. The sheath and core are connected to, respectively, first and second slides in a handpiece. An single actuator first moves one slide, typically the slide to which the sheath is attached. Once the first slide at least partially displaced both slides are moved to simultaneously retract the sheath and core away from the implantable medical device. The assembly includes a steering assembly for steering the sheath and implantable medical device. Some of the components forming the steering assembly are disposed on at least one of the slides. Tensioners prevent the steering cables of the steering assembly from being exposed to excessive force.