88 Ford 250 w/ 7.3 diesel died and i could never start. dont know what happened

so my 88 f-250 with a 7.3 navistar diesel had alot of charging and starting system problems. its has gone through many batteries, starters, and an altnerator. well one day the truck's main power wire leading from the starter realy down to the starter itself grounded out and caught on fire. so after putting out the fire and replacing all the lines, the truck never started again. i replaced both batteries, starter relay, starter, igintion switch, and power wires with the correct gauge, and the grounding wires.

any ideas? i dont have the truck anymore but that is one of those things i want to understand what happened.