Well it does look like a tach. But it also looks like the positive terminal wire got hot and melted the insulation off of it. Does it still work, is actually anybody's guess. All you can do is try it. I can't think of any reason for a resistor to have been on the positive terminal. It might be a good idea to use a fuse inline with the positive connection until you verify everything works. That way if the tach is fried inside, you don't cause any other issues. If fact, I would bench try it first instead of wired in the boat. JMHO
