Vehicle in question has had 2 new aftermarket alternators fitted, and a new battery, still no charge from alternator,

I was called out to take a look, from what I can make out, there are 2 pins at the alternator, one from the ECU, one from the body controller, I understand that the ECU controls the output from the alternator, but what does the circuit from the body controller do?

If I flash a 1amp fused live to that circuit for a split second, the alternator starts charging and keeps charging until the engine stops and restarts

I have various diff wiring diags from E3 and all data, but none state what circuits do what, with a scope I can see the pulse width from engine ECU but nothing from body controller. Does anyone have any information on how this system functions?

I'm inclined to say its because its non genuine part but I'd like to carry out some proper tests first to confirm.