Hey Pete
Finally got off my ar$e and had a look under my bike's seat today (hooray for weekends!).. it's definitly from the alternator, going to the regulator/rectifier. If you have a look at the circuit diagram (avail on my website btw) the wires're listed as being blue/black but they change on the other side of the connector to that yellowish colour.
Theories:
1: Loose plugs can cause resistance. That would fix iteself when you changed the connector.
2: Did you do anything experimental like finding out how much light you could get if you put 150W bulbs in the headlight and driving lights? *grin* That line's not fused so any load greater than rated would overhear the weakest point (almost always the connections).
3: Worst case scenarios are that your rectifier/regulator's shot, or the alternator's putting out LOW voltage (which would result in higher current). That would also mean your battery'd constantly have a low charge, and your lights'd be dim. You might have ignition problems too, especially at high revs.
If it's not happened since you replace the plug I'd be suspecting the first theory, but keep a CLOSE eye on it for a few weeks... having an electrical fire under your butt'd be no fun.
Matt
