Unlikely the mechanic caused the failure. He didn't mess with the electrical system, but you did; and the turn signals specifically. It's a good chance your LEDs have something to do with it. Start reversing everything you've done, check operation at each step.
I've got a Y2K 1150GS, when I installed my LED turn signals I got a "fast flash" when I turned them on, but the hazards worked normally. This is because the flashing is controlled by the motronic. You may find the problem lies therein.
I hope this helps, somehow.