The rectifier needs to be grounded. The black wire coming off the rectifier ties into the wiring harness ground that eventually gets to the main chassis ground. If these wires are suspect, cut them out and solder in new wires as a reliable ground is necessary for the charging system to function adequately.
The pink wire in your blue plug connects directly to your stator. On your bike, the three AC legs off the stator are blue, yellow and pink. You should remove your stator cover. You will note that the three wires mentioned crimp connect to the leads off the stator windings. You should remove the crimps and solder new wires to these leads and connect in place in the blue connector. Many times, these wires fray and short where they come through the rubber oil seal block in the stator cover and this is probably what is happening to yours. The covers get hard and crusty and just deteriorate. You will find that there are also wires going to your oil pressure sending unit and neutral light switch going along with the three AC stator wires going to the blue plug. If the ends up near the plug are good, you might want to cut all the wires and solder new wire to these ends with the plug still attached especially if you are not all that handy putting pins in an OEM connector.