One side of the ammeter IS a black wire. I know. Just go with it. The black wire has the famous welded splice with about 5 other positive wires in it. It looks like something a caveman soldered together, but it's factory. Make sure the ancient tape over the welded splice is still good. If not, remove it, clean it up GOOD and re-tape it with GOOD quality electrical tape. There are some other products too, but I've found as long as you use a good quality vinyl electrical tape, it will be fine. But as mentioned, you're going to need to do some physical tracing, because what it sounds like is, that wire......on one side of the ammeter or the other has found ground when it's not supposed to.
