Agree with Max......first place to check is the starter relay.....when vehicle won't crank over, make sure that relay is clicking......if it doesn't try swapping relay.....still doesn't, take the palm of your hand and hit the fuse block while someone turns the key to the crank position....
The BCM, gets a signal from the ig switch.....it sends a signal to the ECM.....the ECM then powers the starter relay....