2008 GMC Acadia Trailer Wiring - Back Up Lamp Circuit

On my 2008 Acadia, I have a factory installed trailer tow package and hitch. The center pin on the 7 wire plug is supposed to be for the back up lamp circuit. As I understand it, this wire should activate when the vehicle is put in reverse to disengage trailer surge brakes. However, mine does not have any voltage. The manual calls out this condition and says if this circuit is not active to contact your dealer. My dealer's service department has no idea what I'm describing and wants to install an electric brake controller. My trailer does not have electric brakes, only surge brakes that are to be held off by a solenoid in the tongue when the back up lights are on. Can you describe/diagram how to connect or activate this back up lamp signal?