Might seem a silly question to some of you, but what is this I have circled in yellow in the picture.. Reason I ask is that there is a red wire about 12 - 14 guage with a loop connector on it that isnt connected to anything, but is about 3 inches away from this terminal.

local://upfiles/1730/C005F8D6815F4AEBB54A9B056637DBF1.jpg

I believe that is a 12v bus bar. All of those lugs should be +12v hot. Do you have the tow package (factory installed hitch)? If you do, then there is a wire harness that is pre-wired for an electronic brake controller and that wire is the main power wire for the trailer brakes.

Was there ever a plow or winch on the Blazer?