You would get better help on one of the schooly sites. Folks on this site are into highway coaches, although there are many good mechanics here, we are not very familiar with school buses. Good luck fixing your bus.
https://www.skoolie.net/forums/f5/
ECM will be mounted on the engine, along withe IDM and the EGR driver.The ESC (Body Controller) is mounted in the left side kick panel. Its wiring comes through the bulkhead on the left side. It may have 1 or 2 heavy red power leads. The will come from the Mega Fuse (near the M/C). The ground will be on a stud on the bulkhead.I would start in the battery box and work forward, The "clean power" harnesses are a known issue, I would wiggle them a bit and see if that helps.