Had an 2003 Ford F-350 with a v-10 engine come in with an electrical issue would shut off while driving down the highway and not want to engage the starter all the time.
Spent 3 hour Diagnosing and repairing it removed panel from under steering column and dropped fuse panel with GEM module on it down to check the voltages. By doing voltage drop I was able to determine that the proper amount of power was not getting to the switch and fuse panel.This was due to a corroded connection between battery and fuse panel and switch a connection like this will supply enough voltage to light a test light or show battery voltage on a meter but you will not have enough amperage to support the amp draw of the circuit.By doing a voltage drop test it will show a problem with the circuit.
wire on left side is from battery right side is going into cab of truck
another view of connector