Battery isolator

Wondering if anyone has an idea how the batteries are charged on our boats? Sounds like a stupid question, I know. But drawing on memory, I remember there was no specific charging cable going back to the battery from the alternator. Researching with the PCM engine manual, the diagram shows the alt charge cable leading back to the 8 pin connector, then nothing after that. It appears that the charge back-feeds through the main cable? Or is it that one of the PO's has the wiring a little screwy??

The reason I ask is I'd like to be able to simultaneously charge the deep cycle while running on the the starting battery and the battery selector switch is only set to the main (starting) battery. But the isolators require a specific charging cable from the alternator, which apparently I don't have.

I was looking at the same diagram - that's what I thought I saw.   Last one I installed was on my old boat - GM 496, so maybe it was different. But it had an isolator already and the alternator cable went directly to that and then the batteries. When I added a 3rd battery for the sound system I had to get a new one that had 3 outputs. With this Ford system, I'll just have to run a new dedicated cable from the alt to the new isolator.