I got my 2012 DL650 really dirty the other week and so I gave it a good wash. Everything was fine but the next morning the battery was dead.
By the time I got around to messing with it I discovered the left turn signal was stuck on. Not blinking. Just steady on along with the dash indicator. As soon as I put power the bike this happens. Yesterday I poked around trying to see if anything obvious was loose or would change the behavior but nothing I do short of pulling the main fuse makes any difference.
Now I am going to take out the battery box to get better access to the wiring and check that area for shorts as well as try to examine the wiring inside the cowling. I have a wire diagram which indicates there must be a short to a black wire somewhere but that could be anywhere on the bike from front to rear.
I just thought I'd post to this forum before I got started in case anyone has any tips or words of encouragement. Of course this has to happen as soon as the weather changed and I was ready for a whole weekend of fun.
I'm seeing, in your report, some seemingly contradictory (or at least puzzling) observations. You say the battery went dead overnight, but presumably the ignition was off. Per your "left signal was stuck on ... as soon as [you] power the bike", it would seem that the overnight power drain was not the turn signal. So, either the symptoms have been changing, your report is missing something important, or there is another faulty drain other than the errant turn signal. Can you clarify?
It would be a strange short that goes from the switched (by ignition key) DC bus to the post-switched turn signal. Given the current needed to illuminate the stock incandescent signal bulb, I do not see simple, water-borne contaminants as a likely cause. Could something metallic have been carried into the turn signal switch? (You did not pressure-wash the switches or parts of the wiring harness, surely!)
I would also point out that the wiring, between where wires are terminated, is quite waterproof. It has PCV insulation. So any short you are likely to find should be at the terminations. (There are edge cases, such as chafed-through insulation which your washing jiggled enough to produce a short which was incipient anyway, but having a couple wires do that with each other is a real stretch toward the improbable.)
You say "nothing I do short of pulling the main fuse [keeps the turn signal from misbehaving.]" Does that include pulling other fuses, individually?
As for encouragement, I can only say: Using a wiring diagram is an excellent step in your trouble-shooting process. This is probably going to be an unusual problem for which some more symptoms and experiments will be needed to sort out what has gone wrong. Do you have a DVM or continuity tester?