Well, the "Brake" lights should not come on with the headlights, the rear parking or running lights should be on. Need to know what year the car is. THe brake lights are dual filiment bulbs.
On to the problem. A dim light is most likely a bad ground, a corroded contact on bulb socket and/or the bulb itsself.
ALso, if someone has done any modificartions to the wires in the trunk, it's possible that the wire of the brake light and the parking light got switched on one bulb, and you are lighting a parkng light filiment on one side and a brake light filiment on the other side-one is brighter than the other inside the same bulb...