2000 Dodge Truck brake lights won't come on

It is likelly something to do with that wiring in the back. I have seen that before where the only way to get any of the lights working is to hook up a trailer or hit an odd switch. The harness for a trailer is hooked up in paralell with the lights on your truck. Both get power from the same place, but they are grounded in different spots. When you hit the trailer control the power is looping around to a trailer ground. It is likely a bad ground for one of the lights in back, Check anything though as there my also be a short somewhere.