So it does sound like he tried to go around the OE harness...the good news is that you have the OE wiring diagram to fall back on.
Yeah, electrical sucks. I really feel like a contortionist every time I try to work under the dash, but that is in large part because I'm usually too lazy to do the right thing and remove things in order to make my world more comfortable.
Is your 4bt 12 or 24 Volt?
If I understand, Mongoose has got a 2H with 24V in a FJ60 USA body...so it must have been a conversion, not diesel from the factory, thus the converter from 24V to 12V for the 'appliances'...