I heard that there is a recall on the tail lights of some of these Mustangs, and that may be part of your problem.  You might want to check with the dealership to see if in fact, there was a recall on the tail lights for your 1999 Mustang.  Otherwise, there could be a problem with the wiring.  Do you have the sequential tail lights, I wonder, and is that part of the problem.?  This link has information about the tail light issues. http://townhall-talk.edmunds.com/direct/view/.f0fd78f.  You can go to car.com/Ford and see if there have been any recalls on your car.  This site also answers questions about brake light and tail light problems.  See if any of this information helps and if not, post on the sight and they will answer. http://www.hotrodders.com/forum/brake-light-turn-signal-tail-lamp-problem-77857.html