It should show + . On the engine side of the fusible link it should start at the battery side of the starter relay, from there through the fusible link, and then to the dash to the positive side of the amp meter. The other side of the amp meter (black) should go to the battery terminal on the back of the alternator. Either side you should have a positive reading on your volt meter.
Disconnect the battery, switch to ohms and see if you get a reading between the back of the alternator and ground. You shouldn't get one.. then do the same on the starter relay (battery) to ground. With the amp meter disconnected on one side. If there is a short then it will be on one side of the amp meter therefore it will narrow down where the short is.