This is one of the top ten questions that noobs to trailvoy and gmtnation ask. Actually, it generates about one or two new members every week! For years.
Anyway, if you attach the ring lug on the mystery red wire to the adjacent threaded stud and install an M6 nut, it powers up a pin in the rear 7-pin trailer connector that recharges trailer batteries while you drive along. Many folks think it's involved with the electric trailer brake harness, but it's not. It's forever unknown WHY the GM designers left the ring lug off the stud, but there are a couple of theories.
Anyway, the stud (and my M6 wingnut) is shown in this picture that's been posted a hundred times on the various sites (including here, I think). Mostly it explains how goofballs blow their megafuse in a jump starting exercise gone horribly wrong.
