Well unfazed answered already while I was writing so there's no point posting this it took a while to write so I'm still gonna post it
I'm assuming this is the wiring diagram for your bike?
http://foc-u.co.uk/index.php?action=downloads;sa=view;down=5 I think the fault is corrosion/a bad connection on the chocolate wire causing high resistance which is causing a voltage drop and limiting the current to the bulb. I think the fact that when you remove the left indicator bulb the arrow starts working shows the problem is in the chocolate wire. If the problem was a bad connection at the indicator bulb, a blown bulb or bad ground at the indicator bulb, the arrow would still work regardless of any of that but the arrow isn't working so that rules out all those problems. I think it has to be a problem on the chocolate wire. It could be corrosion/bad connection in the chocolate wire but corrosion/bad connections are more likely in connectors than wire so that's where I'd start. The diagram shows the chocolate wire going through the connector between the main harness and sub harness 1 at the front of the bike so that's where I would look first. If there's no signs of corrosion in there on the chocolate wire connection it has to be corrosion in the chocolate wire itself.