Jimm 5/28/2012May be related to a fulty brake light switch at the brake pedal. May also be related to a faulty brake light relay - though this same relay usually controls all the brake lights including the center high mounted stop light.
Oxides 5/28/2012i ran across another gm where only the high mount stop light worked. in this case, the other brake lights ran through a different circuit, routing them through the combo switch. the combo switch was getting the signal from the brake pedal switch but not sending power to the left or right brake light. also, some bulbs have more than one filament. so check the bulbs and see if they look good. if no lights work then the stop light switch could be bad, or it could be a fuse. start working through the simple things first.
HouseCallAuto 6/1/2012the turn signal switch (combo switch like Oxides said) is junk. probability here is greater than 90%. Hi mount stop light bypasses the TSS, separate circuit from the rear brake lights. This is not that uncommon as I have dealt with this before once or twice over the years, always on a GM truck.