when my headlights are on my breaklights do not work

Hi, my name is Devin. I am an auto repair expert with the Car Genius team here at RepairPal. So sorry to hear you're having trouble with your vehicle. The issue you're describing with your K2500 suggests there might be a grounding issue in your brake light circuit. When the headlights are off, your brake lights are able to find ground through the headlight circuit. Checking the brake light circuit ground connections for corrosion or loose wires and inspecting the wiring for any damage would be a good start. If you need more advice, have any other questions about your vehicle, or would like to get connected to a RepairPal Certified shop near you, we have you covered. Give us a call at 877-913-1554, and we will happily help you out!