In some examples, an extension includes a connector adapted to electrically couple to a proximal end of the therapy delivery element. An elongated extension body is attached to the connector. The elongated extension body includes a stylet coil having a stylet coil lumen. The stylet coil extends within the elongated extension body to the connector. A conductor assembly includes a plurality of insulated electrical conductors braided to extend around the stylet coil and electrically coupled to the connector. The conductor assembly includes an inner lumen with a diameter greater than an outside diameter of the stylet coil, wherein axial elongated of the elongated extension body reduces the inner diameter of the conductor assembly. A low durometer insulator extends around the conductor assembly. A stylet is sized to slide freely within the stylet coil lumen during implantation of the extension in the living body.