The diagram would be super easy. You'd see which supplies weren't sucking a buttload and fusing etc. Then tap off that based on what you want to run, put that (if battery supplied and not ign) through a relay that is triggered by any ignition on wire in the fuse panel.
If would help to a degree if you said what you were looking to run from this extra sub panel.
Personally I would never run power for an IC pump or EWP pump through the firewall. I'd run a ground on/off trigger only. That way if anything happens to it, the default is on with ignition. Those relay holders I mentioned would give you a nice install. Or you could hide a weather proof relay down in the bumper with the pump.
If you are looking for a kick butt relay with some nice 10gauge on it. Hit up a 92-99 Bonne, Start at the fans and bring that wire back to the relay center on the firewall. You'll have 8 or so feet of awesome wire etc. This is what I used for my EWP and IC. You literally couldn't tell that they weren't stock. But then it had a relay center.
