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 connect or 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 pre-testing prior to assembly onto an implantable medical device.