just fixed mine
had the same problem in my 2002 zx2. It is your brake light switch...or the bumper that operates it.
look at the top of you brake pedal, there should be two buttons installed under your steering wheel, pointed down. when you are not pushing the brake, so it is at rest, the pedal should be high enough up to depress both of these buttons, which is the "off" position.
the button on the left side of the brake pedal (on mine, @ least) was the cruise control switch, the right side is the brake light switch. Take a standard screwdriver and push it between the brake pedal and the right hand switch until it depresses the button as far as it can go, then go around the car and check the brake light. If it is still on you have a faulty switch, 8 bucks at autozone and really simple to install.
However, if the light went off, the switch is fine, and your pedal isn't depressing it far enough. this is what happened to me. There is a small rubber bumper that fits into your brake pedal, extending it "up" enough to depress the button. Mine had cracked and broken off, leaving the switch extended in the "on" position, draining the battery.
no auto parts stores sell the bumper, so you'll have to either a) steal it off the cruise control side or b) get one from a junkyard.