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