Brake lights won't work when the headlight is on

It sounds like you have a poor electrical ground in the brake light circuit. It could very well cost $200 to fix such a problem.

it does sound like a ground but i have found it to be in the fixture that holds the bulb. if you check the wire it probably has ground in the wire but not to the bulb it,s the plug inside the fixture that's not working. The best fix is remove the ground wire (normally pulls out) there should be enough room beside the ground tang where the bulb was to drill a small hole big enough from inside the bulb area trough the plastic to back to put a wire through like 18gauge and use a solder gun to solder the wire on, works great. soldering does require some talent so tip solder (tin) the wire first then pull it back to the metal and set it. if you get to much solder on the bulb housing a small file will take it down. you can find witch wire is the ground by turning on the lights and turns and check to see witch ones they are. of course turn of the lights and turns before you start if the bulb housing/s unplug that's the best way. setting down solder job. use the 18 gauge wire to find the right drill size. have fun