2000 Ford Excursion Short Draining Batteries

I came out to use my car, and when I turned the key, the engine (diesel) barely turned over. AAA to the rescue! Drove the car to my mechanic and after two hours told me that he needs to research the problem.
He narrowed it down to the #2 repay plugged into the interior fuse panel. There was absolutely no drain when he removed the #2 relay, but at this time he has no other ideas.
Can you help?
I'm very mechanically inclined and would love to find the source of the short that the ##2 relay is feeding juice to.
I'm crossing my fingers. Please help! Thank you