So you got the merc a new battery and she died on you. I'd personally check the alt for 12-14 volts, then maybe the multifunction switch (blinkers) for some sort of ground problem. Then clean the switch and reinstall, this kinda happened on my Ranger but that just turned the blinker solid with lights. But sounds like a ground problem to me or sadly a alt. But if it's the alt you might have 2 good batteries just charge them both up. Or I could be wrong, that's the fun with cars.