Utterly crappy paint drawing (no mouse, only touchpad available is the only defense I can
raise....
empeg takes power from +terminal of right battery. left battery is original.

I'm not sure what to tap for into for the control of the relay - possibly one of the terminals on the alternator. The relay should come with instructions... As the alternator spins up as the car starts, a voltage shows up on the terminal which causes th relay to close and both batteries can be charged. As the engine sops, the alternator stops, voltage disappears from termonal nd relay opens. 2nd battery now only powers empeg or other stuff you rewire to it istead of how it's wired now