Where's the main fuse box ? under the hood back by the fire wall ?
well know aera for water intrusion. or mouse damage. look there, I believe relay lives there.
testing & visual inspection.

 

 

Eric at South Main Auto just happened to post this this morning.
you should follow this YouTube channel is one of the top diag guys without getting to Technical

 

UP date , i thought i did everything i could do, so i called a west virginia dot diesel mechanic ,he said the same thing you all said, check connections, battery condition, etc. ask him how much to come to me to look at it, he said he doesnt do much electric work. i got the starter diagram out, started back tracking, starter solonoid, to relay to fuse to battery control module , to battery, i did everything but bcm, i took the cover off bcm loosened and tightened two nuts, cleaned the connectors, and the truck started right up. this mighty help somebody , thanks for the advice.

 

Seems you have it solved, so this is more for others reading for their issue.

1st as mentioned remove belt - AC or other could be locked up or creating a lot of reisitance
2nd On my 6.0l the crossover wire for the batteries corroded in the middle where it was hidden from view. So a continuity test while wiggling it.or at min a visual inspection would help find it.

 

update, my problem was still there, talked to a mechanic, told him everything i did checked continunity on wires , checked relay , fuse, pcm, bcm, grounds. he said sounds like a bad starter, i took the starter out again back to logan where my wife got it, through her cousins friend, cousins friend lets call that strike one. logan is 50 miles away. they set it on the bench , hooked 12 volts to it and it engaged, they said its fine. installed it for the second time which is a job. started once, then just clicked. called my stepson who is a subaru mechanic, he said lets bypass everything, so i got some 10 gauge wire,60 amp push button switch, hooked it from the hot side of the starter to the switch to the control side of the starter. started three times then just clicked. he said the starter has to be bad. i live in west virginia and have to go to montana in a couple days. my wife is in tennessee working so i walked a mile to autozone and bought another starter and took the other one out again , and put the new one in, and it starts just fine everytime. so i paid 185.00 for the first starter, 100.00 to one mech to come to my house to look at it, 50.00 to my stepson to drive here and try to fix it, 240.00 for the second starter, 50.00 for the wire and switch, i dont know how much time taking the tire and inner fender on and off, been 2 months trying to fix this. took the starter back to logan, they said well its been to months and installed we cant take it back now. i said its defective, they hooked it up to 12 volts again and it engaged. said starter is fine, finnaly after threatening to throw it thru the front window they gave me my money back, i also showed them that the small bolt on the starter was very loose making a intermitent connection. so i learned, dont trust your wifes cousins friend who knows someone who can get a deal for you, my 185.00 starter cost me 440.00 dollars. thats my rant for the day, everyone of these wv hillbillys know someone that can do it cheaper or get you a deal, i been here two and a half years, and havent seen it happen yet. but these are overall very nice people who will help anyone, and this state is beautiful.