Solar Panel, Unmarked Pigtails, which is Positive, Negative?

Look very closely at your MC4 connectors. Most have a + or - molded into the body. It's really only an issue if you start making your own cables as I did. But checking your work as well as the manufacturers work is always time well spent.

What makes the MC4 connectors extra confusing, is that the gender of the conductor pins are opposite of the connector body. So when someone refers to a M or F, you are left wondering if they mean the gender of the pin, or the body. But it's all part of the fun.