What could be the problem when only one brake light works

But when I turn on the headlights, the opposite light works? I just bought this car Friday and I'm doing some work to get it road worthy. When I press my brake lights, only one side works, and the one in the middle of the rear windshield. However, when I turn my headlights on and press the brakes, the original working brake light does not come on and the opposite side works instead. Does anyone have any idea what the cause of this problem may be? I was thinking the electrical system may be shorting out or be grounding to the frame due to an insulation failure on the wiring, but I'm no automotive electrician. Any information would be helpful, thank you.