1989 Mercury 80 charging question

[travelinman31]

1989 mercury 80 3 cylinder 2 stroke, tach not working and doesn't seem to be charging. I haven't reversed polarity by mistake, but I may have changed the switch on the batteries while running. I know for sure I ran on the both selection on the switch (due to lack of knowledge and my first time running the boat) and both batteries were fully charged at the time. The boat runs fine, but no tach, and battery gauge never gets above 12. Is the rectifier and regulator the same part? I can see an electronic box with circuit boards and 2 yellows, a red and a black I think coming out of it (sealed connections. Those wires go to posts on opposite side of motor where stator wires come to (2 yellows). They are under same terminal as stator wire (yellow from stator on with one yellow from rectifier/ regulator other yellow from stator on same lug as other yellow from rectifier/ regulator and then grey wire by itself on terminal labeled tach). There are no connections or splices between stator or rectifier, is this where I would test each component? Isolate yellow wires and check continuity? Any help on readings I should see are appreciated. Just learning boats and this site helps a lot. Would switching between battery 1-2-both while running cause a problem? Never switched to off while running. Would running on both batteries while batteries had sufficient charge to do the job individually cause any of these problems?