Going to have trailer brake control added 8/10 at trailer dealer. If my memory is correct, the extra plug and wire that was in glove box when Acadia was delivered is for trailer brake control. The plug is hidden under the left side of steering column, and the plastic housing must be removed for access. For installation, does dealer simply hook up brake controller using this harness and plug? I assume that this plug is already wired to the plug at the hitch, and once the controller is attached, then it is ready to go? Also, what is the exposed plug used for that is visible at the lower left of steering column? Ay help will be appreciated! ???