The part used can be fully modeled or a simple representation of the part. There has to be some geometry to designate the connection points, or pins. For example, the part could be a simple plane with work points representing the pins. For a group of pins, the part could be a simple plane with a single point representing the start location for the group. These pins are the attach points for the wires in the harness assembly.
Content Center contains some generic connectors in Cable and Harness 
Connectors. You can create, author, and publish your own connectors to Content Center.