1983 GMC Sierra 2500 Alternator

Im sorry this question is off subject but it seems that this the only place where any question is answered quickly. Im working on a 1983 GMC Sierra 2500 6.5L Diesel pick up. It has two batteries connected in parallel, im assuming for a plow it has. The problem it has is with the alternator. I have changed the alternator 2 times and it stills continue not to charge. Im sure it is something with the alternator because I disconnected the + terminal of the batteries and the car stalled out. On the alternator, there is only a BAT terminal, the ground which grounds with the chassis of the car, and female plug where a plug with 4 wires going to a little metal box connects to. This little metal box sits on the alternator, im assuming its the voltage regulator since the car is really old. The only thing i havent tried changing is that metal box. Does anyone have any ideas...? Thanks a lot.

That year of truck should have a 10si or a 12si alternator.  Those have an internal regulator stock, but it might have an isolator hookup.  Is there a battery in each of the front corners of the truck?

That year of truck should have a 10si or a 12si alternator.  Those have an internal regulator stock, but it might have an isolator hookup.  Is there a battery in each of the front corners of the truck?

Dual batteries were a stock feature on the deisel trucks, they needed a good bit of cca's to roll over the engine initially.  Seems like the isolator is bunk, and isn't letting the batteries charge.