1996 FLD120 what is this

thanks, ill do that too. totally forgot

 

im a car mechanic, ive put on ring terminals many times, im just not the best at electrical diag

 

alright update time: I cleaned every wire in the battery box using emery cloth, I replaced the fuse holder in the battery box and fuse, I rewired my tail lights in the rear, I put in the new positive battery to starter wire, and hard wired the b3 pin of the ecm to the battery direct also put a fuse on that too. Truck wont even turn over now. I hard wired the b3 pin of the ecm after i found the truck not turning over. i tried lightly tapping the starter to see if it was that and it still wont turn over. im lost now. also my batteries are at 12 volts and 3/4 full, i left them on the trickle charger over night.