A modular header and method of fabricating same for making electrical connection between an array of feed-through pins extending through a wall of a hermetically sealed enclosure of an implantable medical device and lead connector receptacles within the header in which the header is fabricated using a pre-formed molded header module, together with a set or harness of interconnected flexible conductors incorporated and sealed by an overlayer of medical grade polymer material. The assembled modular header is capable of complete pretesting prior to assembly onto an implantable medical device.