I was writting about that blue wire test too, but if the engine runs, the posibility about a non energized blue wire at brush is hard to happen since the circuit feeding the reg and brush is the same than the ign system. EXCEPT if the wire between the ign circuit splice down the harness tape and the brush is broken. I only can see that fail right at the field terminal but not down the tape.

Ammeter circuit fail? Thinking on the year, it should be a shunted system, where the full load between alt and batt is not running anymore throught the ammeter. Not even into the cab.

Nothing wrong on check the blue wire status thought!