Break lights work like normal untill i turn on the headlights then the left side does not. replaces bulbs, still does it.
It sounds like a bad ground wire for the left side break light. You will need a meter to see which wires have power to them and the wire with no power is the ground wire. In US cars it's sometimes black. In European cars, sometimes blue. You can check other devices like the dome light to see which color wire is used. Once you determine which wire is ground, splice in a new ground. When you have a bad ground, electricity will back feed through any path to get to ground. Yours is back feeding through the headlights. I have also seen it back feed through the turn signals which lights up the arrows on the dash.
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