Rear tail lights do not work. Went out. What to do?

I have cleaned up the ground wire that screws into the metal bracket at the rear of the trunk. I even used a separate cable to make certain it was not the wire. It is grounded. Anyone else know where to look? What happens when the flasher goes out? I assume the light goes on, front and back, but does not blink. This one only the front comes on. No back lights. (except for rear side/running lights)