I had a similar thing happen with the rear turn signals. The issue was that I swapped two of the wires that went to the back end. What would happen is that when I pressed the brake the rear turn signals would turn on, but the license plate light would turn off. The brake light functioned like normal. If the license plate light didn't turn off, I would have kept it like that, because extra brake lights aren't a bad thing. Getting the correct color wires connected fixed it.