Headlights dim when push the brake pedal

[REByar]

Hi All,

I just installed an additional drum style brake light on the right side. The running lights work, the brake lights both work. But when I am driving at night and I put on the brakes the front lights dim. Just for a second or two and then they brighten back up. Is this a ground issue? When I just had the original brake light on the left this did not happen. Just started with the additional new one. I use an alternator and the system is still 6 volt.

Any assistance would be greatly appreciated.

REB
August 28 Tudor