So after using the stereo, and running the battery down, way too far, I figured I may need to put in some sort of low voltage shut off. I was also planning on wiring up the voltage meter, but I cannot find it. So I bought another one, and while I was at it, I bought 2 of these waterproof plugs, for the speaker connections.

They are overkill, rated at 30A at 500v AC, but they should work.