Is there a grren wire in the r/r harness, I don't see one in the pic. If not: the case has to be securely grounded, generally through the mounting bolts. make sure these go into the frame or a solid bracket screwed to the frame, not a clamp. You can use a ground wire... I use ring crimp terminals attached using an external toothed lockwasher between the ring and the r/r case, and between the other end ring and the frame. That washer pierces any paint or anodizing and makes a solid connection.
