High Mounted Brake Light Not Working

I am new to this site and to owning a mustang so i hope you'll forgive my ignorance. I have an 04 V6 and have been trying to get the high mounted brake light to work. From what I understand from the owners manual and chiltons book the brake light is on the same fuse as the trunk light and license plate light which both work. So i replaced all four bulbs and the brake light still wont come one. I'm not sure if it suppose to come on when i have the headlights and running lights on. i had some one watch while i held the brake down and it wont lite up. this was after i replaced the bulbs. It was suggested to me that the sockets maybe burnt out. I tested them with a dmm but couldn't get a reading. I'm either doing it wrong or the while thing is bad. Not sure what else to do and don't want to spend more money on parts until I know what I'm doing. Any advice or tips would be helpful thanks