This was the first problem that I fixed on mine. You will also notice that the rear brake lights don't light either because of this.
It is the rear brake switch that is causing the problem. If you look at it on the rear diff on the left hand side. You will see a spring attached to it. I found that the spring is not strong enough to pull the plunger on the switch out. If you just reach in there and pull it gently in the direction that the spring is pulling it should start to work.