This thread does have something to do with my question since it is a diesel install. Diesels already have two batteries. I was asking which battery he used.Well honestly, there's nothing in this thread that has anything to do with your question. Not sure if you posted in the correct thread, or let it scroll down to far, and ended up in a different thread.
If you are putting two batteries in, and they are not on an isolator put it on the second battery, the one that's not to the starter.