RDT

My son's '99 V6 Mustang has been having a no power issue, and we narrowed it down to the cable that runs from the battery to the fusebox being loose where it connects to the fuse box. No problem, I'll just tighten that up - WRONG! It's just spinning, like whatever it is connected to on the other side is stripped or broken. So how do I take the fuse box off to see if I can to the back of the bolt> I removed the one bolt I could see on the inner fender, but it's still attached to another metal bracket in the back, and I can't find any bolts holding it in. Anyone have any words of wisdom?