Could be one of two reasons metioned.
Water from the A/C drain is either blocked, or being pushed back into the cab, best fix is to attach a 5/8" 90º bend to the water drain and then attach a 6-10" hose to the 90º to make sure if drains to a lower location.
The other is the 3rd brake light. Just remove the light and replace the gasket, a little clear silicone probably wouldn't hurt as long as you're neat.
I had BOTH problems, fixed one and it got better, fixed both and it stopped completely.