Fitted the Shindengen FH020AA MOSFET RR, with massive gauge wires, direct to the battery via a BIG in-line 30A fuse holder, and direct to the yellow stator wires, cutting out the stator connector just behind the tank.
All working lovely, 14.6V at high revs and 13.6V at tickover with headlight on. Doesn't change much with headlight off, just 0.2V more....which is good regulation.
This was the corrosion behind the OEM RR where the earth wire eyelet attaches.
Maybe that was the original problem if the VTR RR dumps current through that auxiliary wire rather than the ground wire in its connector?
Anyway... the Shindengen is a much better RR, so a worthwhile upgrade.