1994 Ford Escort break lights

Im having a problem with my break lights. Most of the time they don't work, but every once in a while ill look back when I break and ill see the reflection of my break lights on my rear view mirror. I have tried ruling out problems with the swich at the break peddel by jumging the leads and bypassing the pedal and I got nuthing. Im worried that that means that I have a short or an intermitent short either from the power sourse or in the line to the lights. Im not sure what to do or what I should check if you have any ideas please help. Thank you
steve