1995 C1500 brake light problem

The rear brake lights on our 1995 C1500 keep going out. We recently replaced the turn signal multi switch and the brake light contacts on both sides because the lights went out, and the switch and contacts overheated and shorted out. Now, just 6 months later, the lights are going out again. The overhead brake light on the cab works fine, the turn signals work, and the running lights work, but not the brake lights.
Any ideas?