If you have a look on the rear fender you'll see there is an oval hole on its top under the seat. There is where the fender harness must go out and connect to that mysterious plug 4.
@Ya Tigor@Venom0852 see if the colors match 7A at the bottom right of the schematic.
Maybe you can get the wire colors on the connectors and look at this schematic and figure out which one it is. Once you know which number the connector is, the Electrical Diagnostic Manual can give the location of the other half.