just generally considering the question of more lights on the system being dimmer or not, and with respect to a "3rd brake light", which wasn't part of the option ... for instance, the wires that go to the points the extra lights get power from, supplied 1 brake light of whatever wattage, now that same wire needs to supply 3 lights of the same [or more?] wattage ... just as the gauge of the battery cables is fairly critical on a 6V, the gauge of wires further along will be as well, and maybe the brake lights are higher wattage than the [stand alone] turn lights?
Bill