Wanting to hook up a 200amp isolator to separate two batteries. One lead has to go to a true ignition source.

I want to use an add-a-circuit and plug into the fuse box underneath my steering wheel.

Diagram:



Of the fuse locations listed, what would be a "true ignition" source? I'm guessing that is just a 12V ignition line...i'm guessing the radio would be ok for that, or the ignition coil?

Thanks