Yes that large red cable in the photo goes from the fuse box to the positive of the battery ,it is un-fused . I don't think its shorted to ground but i just wanted to be sure by disconnecting it from the fuse box. I tried testing the cable with a Ohmmeter for resistance to ground but it doesnt read much, it appears to be a capacitive type load which seems to indicate that the real load only appears when the BCM is activated ( i.e powered up ).
Yes ,as you say, my plan was to isolate the problem to the integrated BCM/fuse box and then unplug each of the connectors one at a time but im not sure if that is the right thing to do while the BCM is still activated ? The drain does not appear when the BCM is not activated / un-powered , so im at an empass .If i could find some info on what each of those plugs were for i could do some continuity tests to ground, as you say.
My hunch is that some of the connectors are for sending ( i.e. coming from the BCM ) and some for receiving ( i.e. going to the BCM ) but i'd really like to find a wiring diagram of each of those connectors with some associated info that would be of great help
:/

 

Found more info on that large red cable : Its a #4 gauge wire , the connector end is a C2280G ( f150's 2009 -2014 12th generation) single pin and it is the positive supply for the BCM and fuse box./
Link here :/
https://static.oemdtc.com/GSB/G0000071.pdf

 

On my king ranch 2006 it turned out to be my starter cable was disintegrated inside the sheathing rubber covering the end..I only found it when I moved the wire checking the connection and found it pretty much fell off the starter……worth a look….

 

Yes i did think of disconnecting the main cable on the starter to see if the power drain goes away but i had a new starter installed last summer and had the terminals and wiring cleaned up so i had the starter wiring ruled out as the problem but will keep that in mind when i get a chance to work on the truck again hopefully this weekend.

 

Go under your hood and on the sides of the inside panels have body grounds..there’s about 18 around your truck..check then all…remove…sand down the areas…dielectric gel..and replace….also under the tailgate near the trailer connection under your truck is a bad one….and another just to the left of it….just a suggestion
do a search ground schematic to find out where yours are for your particular truck….

 

what dash lights would come on? The blinker indicators?

 

Since the truck, seemed to develop the issue during a period of disuse, have you considered rodent damage?

 

I would think that big a load would show up in any one of the harnesses being warm to the touch then you can narrow down the wire. Taking the BCM out isn't that big a deal and may save you hours of head-scratching.
I like the rodent Idea too. Could be something shorted on back of the PCB due to rodent debris as well as chewing.

 

There's no rodent intrusion , im glad to report , and i managed to get the main cable feed to the BCM/fuse box unplugged ,it was not easy , removed the red locking tab ( it was only on one side) and had to unbolt the unit
to get hold of the connector. But that did not remove the battery drain , even after disconnecting the main positive cable going to the starter made no difference. Spent another 4 or 5 hours probing fuses, relays and wiring harnesses and found nothing otherthan two blown fuses for the tow hitch connection which i never use anyway.
I was stumped so decided to put everything back together and start over . When i did another load test on the battery negative the current drain had now dropped from 12 amps to 6 amps ! Started disconnecting the main cables again one at a time as before and when i disconnected the alternater cable the draw dropped to near zero, when i connected it back up the current jumped to nearly 16 amps for a split second and dropped back down to about 2 amps and then to zero again. Waited about 5 minutes and retested and the current drain jumped to 6 amps again!! So appears the alternator that i had installed just 2 summers ago gone bad.
Called the dealer he said its now out of warranty but can take a look at it , i removed the alternator today and will be dropping it off tomorrow but im still not convinced thats whats causing the drain problem, im thinking the short to ground may have damaged the aternator ?
I did not see any bad wiring or corrosion behind the BCM fuse box and the connectors look clean no sign of water intrusion:

 

Glad you got it tracked down!
Hopefully, once the alternator issue is resolved you can find out for sure what caused it to go South. Good luck and I hope you post back so others can benefit!