What wire gauge is used in OEM negative / ground cable?

Definitely more common to put those on the positive lead if you can.

Edit: those switches are never used under max load conditions like starting so you don't have to worry about peak current so much, regular battery disconnect switches will work just fine
 
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Just a simple knife blade switch. American made. Had to swap the posts around to make it work. Not intended to be a theft determent, only to stop the parasitic battery draw.

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