Another thing to check. Make sure that the 8400 isn't in app mode when the iPhone is connected. It probably isn't, just thought I would throw that out there.
Well, I switched out the 8400 for the 80PRS just now and the 80PRS works like a charm with the iPhone 5. Directly connected with the USB to Lightning cable it works in App Mode as well. I can't believe it. I think the 8400 is actually newer than the 80PRS. Oh well. I'm happy. It is serious overkill since I have the PS8 but the superior DACs will end up being worth it in the end. That and the better Bluetooth. I hope other AVH-P8400BH owners trying to use and iPhone 5 find this thread before they go through all of the hassle that I did.
Thanks to all who helped!
This shit belongs in the dome.