Battery drain - SOLVED

Well that is odd because that is all supposed to be powered through the ignition switch. Outer terminals are backup lights, center terminal is grounded in park/ neutral and is the neutral switch.

You might have something like a wet corroded harness "let's say" near the trunk, leaking juice between the dome light circuit which is hot, to the backup lamps line going on back to the rear. Just an example.

I guess? you have pulled all fuses and checked?

Also wondering if you inadvertently wiggled something when you disconnected the NSS and "disabled" the draw