Ford 6.0 Battery drains when not plugged in to block heater after replacing the radia

I saw a post on here that was very old but what it said was the batteries would drain and not start unless the block heater was plugged in. I'm having the same issue except it never did this before until we replaced the radiator just a few weeks ago. The weather was the same before and after so it's not colder out after replacing The batteries and alternator test fine and I know there is no connection between plugging in and battery charging. If it's not plugged in they drain very quickly and a jump start works immediately when it's plugged in it starts without any issues. The only thing we can figure is a short of some sort but everything looks ok. If anyone has any ideas on locating the problem or where the short might be I would be very appreciative we are at a loss.

Does the starter spin, and it just not crank without the block heater plugged in? Couldn't follow

I'm just here for the popcorn and to see what in the fluster cluck is going on here. Sounds like a wire got shorted out somewhere to me.

Check ficm voltage output when cranking. My bet is either dieing ficm or tired injectors. Could also be a weak starter.

sootie said:

Could also be a weak starter.

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Thats what im thinking, or a poor connection on a battery cable.
Im assuming that probably at minimum you disconnected the positive cable which ties the batteries together? Seems that something of that sort was disturbed during the radiator swap and is now causing trouble.
Try checking voltage at each battery while cranking engine. You might also check voltage at the starter high current terminal while cranking and see how it compares to your battery cranking volts.