Dash Power Feed Confusion

Hello members, I am having some trouble getting some wiring sorted out on the car, mainly where the gauge cluster gets power from and where my alternator lead plugs in. There is a harness connection at the firewall with a red wire and 2 black wires, heavier gauge, shown below.

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The red wire connects to the red terminal of the gauge cluster and it gets 12v from the battery. The 2 black wires form 1 wire once inside and run to the other peg (ground I assumed) of the gauge cluster. These seemed like they were once run to the alternator for some reason but I figured that couldn't be right as it would have +12v when the engine is running. I cant seem to find any other connection that exists that would make sense. Shouldnt the 2 brown wires from the alternator run back to battery? And should the black wires comming from the gauge cluster run to battery neg. ?? Here is a picture of my gauge cluster backing aswell. Any help is appreciated.

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