Alternator Not Charging House Batteries While Driving

When it's not working, check the voltage of the wire to the NC connection of the boost switch (from a run/accessory circuit in the chassis). If there's proper voltage there, then the problem pretty much has to be the switch itself. If there isn't power on that wire, then trace it back and see where it disappears. If it's too hard to trace, you could run another wire from some other reasonable location...and make a note of it in your literature for next time.