also i know on my truck (1994 k1500 z71) there is a breaklight relay right above the master cylinder for my breaks and i had the same prob you do and it was the relay/weather proof plug on it (i replaced both so i dont know which one it was). also i could get mine to work by pressing on the wires on the relay with someone stepping on break pedel

 

What i did was replace the switch located under the dash. What i found out was that it was moving away from its connection on the brake pedal so what i had to do tape the part that connects onto the brake pedal so it did not move then i got a hose clamp and put it on and it works fine but every once in awhile i do do have to lift the brake pedal with my foot but other than that. Hopefully you have the same problem but it does require two people to check that