I just purchased a used 2020 GMC Denali 3500 dually, the OnStar, nav & gps is not working. There is no power to the Onstar over head unit I've checked all of the fuses that I think would be associated, not sure if the previous owner disconnected something since there is no lights on the overhead unit. Just wondering were to start looking I do have an appointment at the dealership but not until march any help would be appreciated.