I have 1997 k2500 silverado 4x4 with the 7.4l 454 vortec and 5 speed standard tranny. Recently the rear brake lights stopped working. The third brake light up top still works fine as well as the signals, running lights and backup lights. I've checked all the fuses both inside the cab and under the hood. Replaced the brake light relay and taken the steering column apart and checked the power along the harness as far as I could reach without cutting away the metal brakets under the column. I swapped the brakelight switch out with my wife's 98 silverado with no change and her lights still work.

Any ideas on what could be causing the problem would be appreciated

 

Do you have power at the lights? Are the bulbs good? Also check to see if you have a short. If the fuses are good you shouldn't have a short. The wire may be broken. You may have to follow the wire under the truck to see if it is broken somewhere. If you have a test light it may help you find out where the break is.

 

Check your main harness going to the lights. I recently had that problem and found a broken wire